Now it is once every 70 minutes, that is, once every 7/6 hours.
So the actual working time = 8 * (7/6)/(12/11) = 77/9 hours. Over 77/9-8=5/9 hours.
So it should be 8*9+(5/9)*9*2.5=84.5 yuan.
It's very simple. Given the out-of-tolerance part, it is 3 minutes slow per hour, so (10: 50 -4: 30 =)6 hours and 20 minutes, that is, 18 minutes, and 20 minutes (that is, 1/3 hours) is 1.
So the standard time is 10: 50-19 minutes = 10: 3 1.