6/7 of something is only 1/7 less than the whole thing (7/7), so to find 6/7 of 35, we just have to find 1/7 first - which is 5 - and then subtract it from the whole, getting us 35 - 5 = 30 students who play sports.
We can do the same thing to find the number of athletic students who also play an instrument. 4/5 is only 1/5 less than one whole, so we can find 1/5 of 30 - which is 6 - and take that away from the whole to find that there are 30 - 6 = 24 students who play sports and an instrument.
