John buys 1000 balloons. Each balloon holds 10 liters of air. If he buys 500-liter tanks of gas, how many tanks does he need to buy to fill all the balloons? Let's be accurate as possible.
He needs 1000 * 10 = 10000 liters of gas. So he needs 10000 / 500 = 20 tanks.
The answer: 20.