Question

How is 0.9 the simplification to 90/100

Answer 1

The fraction 90/100 is equivalent 1 to 9/10.The fraction 90/100 This is a PROPER FRACTION once the absolute value of the top number or numerator ( 90) is smaller than the absolute value of the bottom number or denominator (100) 
This fraction 90/100 is equal to 90÷100 and can also be expressed in decimal  form as 0.9

Answer 2

Because if you turn 90/100 to a decimal you get 0.90, but you can get rid of the 0 at the end so is just 0.9