If he had 88 as his 1st nine weeks

and he decrease 12.5%

88-12.5 = 75.5

75.5 = 80

The question for this problem is what was Jack's average for the second nine weeks. We know that for the first nine weeks, his average is 88 and his second nine weeks average decreased by 12.5%.

So ... 88 - 12.5 = 75.5his average for the whole 18 weeks would be 81.5%