so, now you have 10^-10/.02 , now you can move the 2 decimal places to the right of the denominator and times the numerator by 100 (since you move 2 decimal places in the denominator) to get: 10^-8/2, which is equal to:
1 x 10^-8/2
0.5 x 10^-8 = 5 x 10^-9