Minor correction on the amount in bold above ^
If you did want 15ml of 24mg/ml, you need 15 * 24 = 360mg of nicotine in the finished 15ml of liquid.
Using 13ml of 12mg/ml + 2ml of 100mg/ml = 356mg of nicotine in 15ml, so 356mg / 15ml = 23.73mg/ml, which is pretty close. About 12.95ml of 12mg/ml and 2.05ml of 100mg/ml would be pretty much spot-on.
Pay attention to Lucent Shadow's calculation:
He multiplies the ml, by the mg/ml. Then you just estimate how much you need to add, and divide the total amount of nicotine (in milligrams, in this case 356mg), by the total amount of juice in ml. If you don't get your desired concentration, try again. This is why we use units mg/ml (which means mg divided by ml).
I'm not great at math, but this is basic stuff. It's the word-problem part that makes it really difficult for us! I need to make a post about this...