problem solving

redhat

--one question from official guide--

For telephone calls between two particular cities, a telephone company charges $0.40 per minute if the calls are placed between 5:00 a.m. and 9:00 p.m and $0.25 per minute if the calls are placed between 9:00 p.m and 5:00 a.m. If the charge for a call between the two cities placed at 1:00 p.m. was $10.00, how much would a call of the same duration have cost if it had been placed at 11:00 p.m.?

ANS6.25

Can anybody solve this?  Thanks!!

tongxun

redhat：
10=0.4*x, x=25minutes. so if a call at 11:00pm, the cost is 25 * 0.25=6.25.