Let the number = x, then you can easily convert the sentence into math/algebra:
If 8 times a number is decreased by 20, the result is the same as 3 times the number increased by 80
8x - 20 = 3x + 80
There, now we can solve for x.
8x - 20 = 3x + 80
5x = 100
x = 20
The number, x, is 20
