He needs 1000*10=<<1000*10=10000>>10000 liters of gas

So he needs 10000/500=<<10000/500=20>>20 tanks

# Answer

20
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
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?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
20
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?
train