I’m really really good at adding up long lists of single-digit numbers in my head. This comes from totalling exam scores. Lots. And Lots. Of exam scores. It’s kind of creepy actually, because I don’t think “Okay, three plus five plus 2 plus 2 plus 9 plus 6 plus . . .” I just stare at the list for a couple of seconds and the answer pops into my head. Even without practice, it’s faster to do in your head than using a calculator–and at least for me, more accurate because there aren’t any miskeying errors, so it makes sense to do it without a calculator.
As someone who does a lot more math on a daily basis than the average population, and there are plenty of things I do well in my head because they’re fast and not too hard and I have to do them often, so I have a lot of practice–but there’s plenty more stuff where I reach for my calculator. How often do you have to do the sort of arithmetic in the OP? I don’t know that it would be worthwhile to practice it, when that’s the sort of task well suited to using a calculator. So I wouldn’t feel bad about it.
What absolutely kills me, though, is that, despite the fact that they are utterly dependent on their graphing calculators and couldn’t add two three-digit numbers in their head if their life depended on it, some of my students are so lousy at using them.
I have students who haven’t noticed that their graphing calculator has an ANS button, so if they need then number they’ve just calculated, they type it in again.
Many of them have NO clue how to switch modes from degrees to radians. NONE. And they’re afraid to go into any of the calculator’s menus, even just to look at them, for fear that they will screw something up.
And scientific notation! Oy! Some of them don’t know what the EXP button does, so instead of typing in 6.4 EXP 6, they type 6.4 x 10^6, which is two more keystrokes because 10^ is Shift-log–or, if they haven’t noticed the 10^ button, they type one zero x[sup]y[/sup]. They don’t know their calculator’s order of operation rules, so when they want 5.2 sin (81+3.2) they type 5.2 sin 81 + 3.2 and get the wrong answer, or if they want 5.2 EXP 6/100, they’ll type (5.2 EXP 6)/100, even though the parentheses aren’t needed. Ditto for 5.2 EXP (-2). Trust me, every calculator on Earth knows that when it sees EXP-2 or x[sup]y[/sup]-3, you want a negative exponent. Considering how often they have to use scientific notation, I can’t imagine how much time certain students have wasted. All those wasted keystrokes HAVE to add up.
If you’re using your calculator more than once or twice a month, it is well worth your time to flip through the manual!
(Except the math profs, of course).