For this answer, you have to divide, so the problem will be 900 divided by 1/10. Since it's division, you flip the fraction behind the division sign and change the division to multiplication. This is called K(keep)F(flip)C(change) cause you keep the first number, flip the second number (only if it is a fraction) and change the division sign.
Now the problem should be:
900 times 10/1 or 900 times 10 which equals to 9000!