Length of path = 3.2 km = 3200 m
Every 400m, there is a seat. Total number of seats = 3200 ÷ 400 = 8
We must add 1 to this value because we must count the one at the start. So total number of seats along the path = 9
There are 2 bins beside each seat. This gives use 9 x 2 = 18 bins.
Every 100 m, between each seat there is a bin. If we start from the first seat, there will be 3 bins between first seat and second seat. Therefore, between 9 seats, there will be 3 x 8 = 24 bins.
Total no. of bins = 24 + 18 = 42
Alternatively, you can draw a diagram to help you. The lines represent the bins and the circles represent the benches.