GMAT Strategy

Building an equation is often helpful in data sufficiency but is most helpful when you are asked to solve for the value of a single thing. Here you are asked to solve for capacity (which means that capacity will be somewhere in your equation).

Then take the sentences one part at a time. "when 200 gallons were removed" - removed is alway subtraction and you subtract what you remove from the whole - thus the equation (using
V for volume) is V-200.

The next part says the volume left (that means the result of the first equation) WAS (is, are, was, were etc.. always means equals. Therefore we are at V - 200 =

then we have "Was 3/7 of the tank's capacity" - the OF always signifies multiplication - so for this part of the equation you have 3/7(C).

Thus the final equation is V - 200 = 3/7(C) - you have two variables and only one equation so you know you need one more equation with either one or both of the same variables and no new variables and you can solve the problem.

So the key to equations is knowing the few english to Math words and then filling in the numbers and taking it slow.

Best of Luck