109-91 is 18, but multiplied is 9919
Simple...
you have:

and

To solve for....

Multiply to remove the denominator-->>

n=12*3
n=36
Then-->>>

Do the same thing-->>

n=-12*3
n=-36
Thus, your answer.
20% goes into 100% 5 times. So you do 480 times 5, which is 2400. So you answer is 2400.