# I'm braindead this AM, math help needed. **URL:** \ **Category:** Factual Questions\ **Created:** [June 2, 2014, 11:50am UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827 "2014-06-02T11:50:17Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![leftfield6](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leftfield6/32/18780_2.png) [@leftfield6](https://boards.straightdope.com/u/leftfield6)\ **Post date:** [June 2, 2014, 11:50am UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827/1 "2014-06-02T11:50:17Z")
I did this same calculation a few days ago, but for the life of me, I have a block this morning, and I can’t remember the right sequence to get this calculation right. Someone with more coffee help me out please.:smack: [ul] [li]I need 20 sales appointments to be successful[/li][li]I successfully get an appointment in 30% of my prospecting calls[/li][li]How many prospecting attempts do I need to make?[/li][/ul] Someone help me out with the formula for this please! I just freaking did this last week, but now I’m just pissing myself off. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [June 2, 2014, 12:00pm UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827/2 "2014-06-02T12:00:47Z")
30% of all your calls = 20 successful calls. 0.30 \* x = 20 x = 20/0.30 = 66.666… Therefore, you need to make 67 total calls. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 2, 2014, 12:01pm UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827/3 "2014-06-02T12:01:18Z")
20 sales is 30% of some number of sales. Is this a homework question? ---
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [June 2, 2014, 12:05pm UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827/4 "2014-06-02T12:05:40Z")
I’m assuming this isn’t homework so… 20/0.30 = 66.66… ---
**Author:** ![leftfield6](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leftfield6/32/18780_2.png) [@leftfield6](https://boards.straightdope.com/u/leftfield6)\ **Post date:** [June 2, 2014, 12:42pm UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827/5 "2014-06-02T12:42:55Z")
No, I’m way too old for homework, believe me. Just trying to help a fellow employee out with his time management. He’s a good salesperson, just can’t keep himself focused. I’m trying to coach him to take a more planned approach. Math skills deserted me this morning. I was reversing the order of where the percentage went in the equation, got it now. Thanks!! ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [June 2, 2014, 1:53pm UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827/6 "2014-06-02T13:53:29Z")
Actually, this problem looks more interesting that simple arithmetic. The best real-world interpretation of “I successfully get an appointment in 30% of my prospecting calls” is that the probability of a call yielding an appointment is 0.3. Thus, it’s perfectly possible that 10 calls yield no appointments (this would be expected about 2.8% of the time). Here’s a link to a [Binomial Probability Calculator](http://www.vassarstats.net/textbook/ch5apx.html) that solves this sort of problem. It yields the following results: Number of calls to produce at least a 50% probability of at least 20 appointments: 66 (52.5%) Probability of fewer than 20 appointments: - with 70 calls: 35.4% - with 80 calls: 13.5% - with 90 calls: 3.9% - with 100 calls: 0.9% And there is no upper bound to the number of calls necessary to guarantee 20 appointments. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [June 2, 2014, 2:20pm UTC](https://boards.straightdope.com/t/im-braindead-this-am-math-help-needed/689827/7 "2014-06-02T14:20:09Z")
is coffee delivery as good as a numerical solution?