so we want something that goes into 400 15 times. 400 goes into 400 once, 800 goes into it twice, and you can keep going counting by 400, so 1200 after that, then 1600. How many times are you going to do this? 15, since. you want something 400 goes onto 15 times. or, 15 x 400.
A small hint for these kinds of problems, lets call that blank x. so what is x ÷ 400 x 400? just x. Then if you multiply the 15 by 400 you get the answer. so for this kind of problem you want to get "x" or the nothing by itself by moving the other parts to the other side. so if something is being added to x you subtract it from both sides. basically you do the opposite of what is happening. This is algebra, and a very very basic explanation of it.
