This will be hard to explain. I understand how histograms work and such but I just don't understand this one

Table:

Time (t seconds): (Frequency beside)

0<t<_10 f=20

10<t<_15 f=blank

15<t<_17.5 f=30

17.5<t<_20 f=40

20<t<_25 f=blank

25<t<_40 f=blank

Complete the histogram. The frequency density is fully blank while the time seconds is partly filled in (missing the 17.5 break).

What I don't understand is how the first one 0<t<_10 f=20 can be 4 boxes high and 10 across (so 4 boxes must be 2). Than 10 to 15 has 16 boxes (so frequency density value of 8) but 5*8 = 40?

This 40 is very confusing because the 15 to 17.5 and 17. to 20 are 30 and 40 accordingly. What have I done wrong?