$920 - $60x
Explanation:
after one month, he try would have taken out 60 x 1 dollars, after two months, 60 x 2, 3 months, 60 x 3, etc. so after x months he would have taken out 60x (60 times x) dollars. if he starts with 920 dollars, he would be left with $920 - $60x.
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