Let BC be x kilometers away.

(x+ 10)/(7.5+2.5)+x/(7.5-2.5)= 4

x/ 10+ 1+x/5=4

3x/ 10=3

X= 10 km

Distance from ship A to place B:

4×(7.5+2.5)- 10- 10

=40- 10- 10

=20 km

A: When the B ship goes from B to C, the A ship is 20 kilometers away from B..