Steps:
1.Pick a number from 1-90
2.Multiply the number by 2
3.Add ten to the answer of 2x your number
4.Divide the answer of 2x your number by 2
5.Subtract the original number from your answer
How many of the numbers you can choose end at 5?
All of them! Any number 1-90 = 5 after you do all this to them or in algebra terms:
x2+10(divide)2-x=5
x= any number 1-90