Suppose that X minutes later, A and B meet again. When they meet, A runs 400x/800=0.5x, and B runs 375 x/800 = 0.46875x ..

When A and B meet for the second time, A runs one lap more than B, and there is an equation.

0.5x=0.46875x+ 1

The solution is x=32.

After 32 minutes, A and B meet again.