Reconstruct a question, answer pair from this explanation: 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.

Q: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?
A:100