Answer:
Bob is at the top, so he starts at step 9.
Sue is at the bottom, so she is at the step 0 (this is the floor, before stepping on step 1)
Sue climbs from one step on one step.
Then she if she climbs n times, she will be at the step number:
S(n) = 0 + 1*n
And Bob goes down by 2 steps each time, then after he goes down n times, his position will be:
B(n) = 9 - 2*n
We want to see when they meet, then we need to solve:
S(n) = B(n)
1*n = 9 - 2*n
1*n + 2*n = 9
3*n = 9
n = 9/3 = 3
n = 3.
This means that they meet a the step:
S(3) = 3*1 = 3
B(3) = 9 - 2*3 = 9 - 6 = 3
They meet at the third steep.
