Need help to solve this Word problem please

Question

(t + 20)(N - 40) = 1300

N = The number of bacteria per square mm, that remains on the hands when washing t seconds hand liquid.

1. How would I express the N as a function of t ?
2. Determine the number of bacteria before washing ?
3. Determine the number of bacteria after washing for 20 seconds?
4. How long to wash before bacteria reduces to 50 per square mm?
5. If to graph what are all the points from part 1?

Answer 1

(t + 20)(N - 40) = 1300

N - 40 = 1300 / (t + 20)

N = [1300 / (t + 20)] + 40

(t + 20)(N - 40) = 1300, set t = 0, solve for N

(t + 20)(N - 40) = 1300, set t = 20, solve for N

(t + 20)(N - 40) = 1300, N = 50, solve for t

You do the graph.