Percentiles and Box Plots · Superchallenge problem

A cumulative frequency graph of $120$ scores passes through $(40, 0)$, $(50, 6)$, $(60, 18)$, $(70, 48)$, $(80, 84)$, $(90, 108)$, $(100, 120)$, straight between. The top $15\%$ of scores earn an A and the bottom $40\%$ earn a C or lower; the rest earn a B. Estimate the width of the B band (A cutoff minus C cutoff).