I'm confused too and I'm supposed to be an expert. I'm not entirely sure what frequency density is. We'll just treat this as a histogram.
Let's assume each five minute interval gives a value equal to or proportional to the number of people that finished in that interval.
From 0 to 50 we have 10 intervals; let's just make this into a table
0-5 0
5-10 40
10-15 40
15-20 30
20-25 30
25-30 25
30-35 25
35-40 25
40-45 25
45-50 15
From 0 to 40 that adds up to
0+40+40+30+30+25+25+25 = 215
From 0 to 50 that's
215+25+15 = 255
The fraction less than 40 is 215/255 = 43/51 ≈ .843
Answer: 84.3%
