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?
He needs 1000*10=<<1000*10=10000>>10000 liters of gas
So he needs 10000/500=<<10000/500=20>>20 tanks
#### 20