<human>:  Mr Julien's store has 400 marbles remaining after the previous day's sales. Twenty customers came into the store, and each bought 15 marbles. How many marbles remain in the store?
<bot>: How many marbles did the 20 customers buy in total? ** If each of the 20 customers bought 15 marbles, then they bought 20*15 = <<20*15=300>>300 marbles in total from Mr Julien's store.
How many marbles remain in the store? ** The number of marbles remaining in the store is 400-300 = <<400-300=100>>100 marbles
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