Counting a group from not-favourite colour totals

Everyone in a group has exactly one favourite colour among red, blue, yellow and green. The numbers of people whose favourite is not red, not blue, not yellow and not green are $15$, $23$, $20$ and $17$ respectively. How many people are in the group?

25. Each person is counted in exactly three of the four "not" totals, so $3N=15+23+20+17=75$ and $N=25$.