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? Think carefully first, then make a decision:
If each of the 20 customers bought 15 marbles, then they bought 20 * 15 = 300 marbles in total from Mr Julien's store. The number of marbles remaining in the store is 400 - 300 = 100 marbles. So, the answer is 100.