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Explanation:
12% of the apples are rotten so 88% are in good condition (note how 12% + 88% = 100%)
Let's say that x is the amount of total apples. Taking 88% of x will get us 66 as there are 66 good apples, so,
0.88*x = 66
To figure out x, we divide both sides by 0.88 which undoes the multiplication
0.88x = 66
0.88x/0.88 = 66/0.88
x = 66/0.88
x = 75
There are 75 total apples. We can check this by taking 88% of this to get
0.88*x = 0.88*75 = 66
and we get 66 apples as expected.