# Math Question...Please Help! **URL:** \ **Category:** Factual Questions\ **Created:** [July 17, 2001, 3:05pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800 "2001-07-17T15:05:39Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [July 20, 2001, 11:27am UTC](https://boards.straightdope.com/t/math-question-please-help/72800/21 "2001-07-20T11:27:38Z")
Actually, I was suggesting a sort of probability/proportion table, not a Venn diagram. Truth to tell, I don’t know how I’d do this on a Venn diagram (or a Vinnie diagram). And yeah, I use a Slide Rule – got a collection of 'em at home, and intend to teach MilliCalm how to use one. But you don’t need it for this problem. ---
**Author:** ![wayward](https://avatars.discourse-cdn.com/v4/letter/w/a87d85/32.png) [@wayward](https://boards.straightdope.com/u/wayward)\ **Post date:** [July 20, 2001, 12:36pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/22 "2001-07-20T12:36:06Z")
I’ll try a Vinnie diagram, using fractions rather than percentages: ```auto ___________________________ | __________ | | _____/______ \B | | / | \ | | | | | | | | | A | | AnB | | | | | \ ______|___ / | | \ ____________ / 0.2 | | ___________________________ | ``` Box A contains the people who got Q1 right. Box B contains the people who got Q2 right. The probability of being in box A, P(A) is 0.75. The probability of being in box B, P(B) is 0.55. Now, since 20% got neither right, 80% must have got Q1, Q2 or both right. So we can say P(AUB)\*=0.8. My Maths book tells me P(AUB) = P(A) + P(B) - P(AnB)\*\* so P(AnB)= P(A) + P(B) - P(AUB) = 0.75 + 0.55 - 0.8 = 0.5 So 50% got them both right. Damn that was hard. A lot of people have got to that answer so I think it’s right. [sub]\*That U is supposed be ‘union’, if you know about that sort of thing. If not it just means the probability of being in box A and/or box B. \*\*The n is supposed to be the intersection symbol. It just means where the boxes overlap ie people who got both questions right.[/sub] ---
**Author:** ![Spectre\_of\_Pithecanthropus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spectre_of_pithecanthropus/32/12343_2.png) [@Spectre\_of\_Pithecanthropus](https://boards.straightdope.com/u/Spectre_of_Pithecanthropus)\ **Post date:** [July 20, 2001, 4:32pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/23 "2001-07-20T16:32:03Z")
First, forget percentages, and pretend the class has 100 students. 20 of them answered neither question correctly, so forget about them[sup]1[/sup]. That leaves 80 who answered either or both. If we add the 75 who answered the first question correctly to the 55 who answered the second question correctly, we get 130, Of course there’s only 80 people, so we subtract 80 from 150 to get the 50 people who answered both. It’s interesting how this question was cast as a probability question, when probability really isn’t an issue. [sub]1. As Herbert Kornfeld would say, they be no-skeelz havin’ wackasses who can’t EXECUTE! ---
**Author:** ![Mr.Child](https://avatars.discourse-cdn.com/v4/letter/m/b19c9b/32.png) [@Mr.Child](https://boards.straightdope.com/u/Mr.Child)\ **Post date:** [July 20, 2001, 5:44pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/24 "2001-07-20T17:44:07Z")
I’m a little late, but I’ll post my reasoning anyway Assume class of 100 people #1 right = 75 #1 wrong = 25 #2 right = 55 #2 wrong = 45 That gives a total of (45+25) 70 wrong answers on the two questions. However, 20 people answered both wrong. Subtract the (20\*2) 40 wrong answers they contributed from the total of 70 to get (70-40) 30 wrong answers. We already accounted for those idiots who missed both, so the 30 wrong answers left represent 30 unique people that each missed one question. 100 people - 20 missed both - 30 missed only one = 50 that missed none. 50 people got both right. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [July 20, 2001, 5:45pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/25 "2001-07-20T17:45:56Z")
**javaman** needs a **slide rule** or something 😃 ---
**Author:** ![Tretiak](https://avatars.discourse-cdn.com/v4/letter/t/a88e57/32.png) [@Tretiak](https://boards.straightdope.com/u/Tretiak)\ **Post date:** [July 20, 2001, 5:51pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/26 "2001-07-20T17:51:49Z")
Just use algebra… A = number who got only Q1 right, C = number who got only Q2 right, B = number who got both right (assume 100 students) A + B + C = 80 (number of people who got a question right) A + B = 75 (number who got first one right) B + C = 55 (number who got the second question right) Solve for B. ---
**Author:** ![China\_Girl](https://avatars.discourse-cdn.com/v4/letter/c/8c91f0/32.png) [@China\_Girl](https://boards.straightdope.com/u/China_Girl)\ **Post date:** [July 20, 2001, 8:37pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/27 "2001-07-20T20:37:16Z")
> [@](#): > > \*Originally posted by javaman \* > \*\*It’s interesting how this question was cast as a probability question, when probability really isn’t an issue. > \*\* Surely probability is an issue, it is just this question can be solved by simple algebra. Funky McDuck’s theory is the best way of solving this kinda problems. ---
**Author:** ![uglybeech](https://avatars.discourse-cdn.com/v4/letter/u/82dd89/32.png) [@uglybeech](https://boards.straightdope.com/u/uglybeech)\ **Post date:** [July 20, 2001, 8:38pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/28 "2001-07-20T20:38:43Z")
PIE CHART, DAMMIT. You’ll see the problem was probably designed to be solved by a pie chart. It took me about 6 seconds to solve it with one. ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [July 20, 2001, 10:50pm UTC](https://boards.straightdope.com/t/math-question-please-help/72800/29 "2001-07-20T22:50:16Z")
> [@](#): > > \*Originally posted by uglybeech \* > \*\*PIE CHART, DAMMIT. You’ll see the problem was probably designed to be solved by a pie chart. It took me about 6 seconds to solve it with one. \*\* I’m curious–how did you solve this problem with a pie chart? ---
**Author:** ![Race\_Bannon](https://avatars.discourse-cdn.com/v4/letter/r/5f8ce5/32.png) [@Race\_Bannon](https://boards.straightdope.com/u/Race_Bannon)\ **Post date:** [July 21, 2001, 12:02am UTC](https://boards.straightdope.com/t/math-question-please-help/72800/30 "2001-07-21T00:02:14Z")
Probablity is not mentioned in the OP, and appealing to probablity theorems is not necessary for solving this. But, it happens that the algebra needed to solve it matches with concepts commonly used in probability, and is an alternate way of approaching it. You could change it into a probability problem by changing just a few words, and would come up with the same answer. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [July 21, 2001, 6:07am UTC](https://boards.straightdope.com/t/math-question-please-help/72800/31 "2001-07-21T06:07:22Z")
**What kind of pie?** I thought in math you used **pi** 😉 [Previous page](https://boards.straightdope.com/t/math-question-please-help/72800.md?page=1)