What is 90 is 1/10 of what
Taking a number's 1/10 means dividing that number by 10. Therefore when you are asked to find the number that was applied the 1/10 to produce 90, you simply multiply 90 with 10. Thus the answer is 900.
\(x \ of \ \frac{1}{10}=90 \\ \\ x*\frac{1}{10}=90 \\ \\ \frac{x}{10}=90 \\ \\ x=10*90 \\ \\ \boxed{x=900}\)
