Since David swam 4 times as far is Yvonne, he swam 4 times more than 0.18. 0.18 times 4 is 0.72. If you want/need to do it in your head, just imagine 0.18 as 18, and that multiplied by 4 is 72. then move the decimal point back over before the 7, to get your answer.
Two of the other choices are clearly wrong because the actual answer is way less than the other choices (except for B). One way to quickly figure out which two (or sometimes three) are wrong is by rounding 0.18 up to 0.2. You can try this quickly because 0.2 is barely bigger than 0.18, and 0.2 is easy to multiply by 4. The answer you would get from multiplying 0.2 is not exactly correct, but gives you a general idea.
