Answer:
24 people
Step-by-step explanation:
((The numbers are up there, so I am not going to define each variable.))
For starters, there is no overlap between the double facilities groups, except the triple facility users, so:
19 + 9 + 16 - 4 = 40 people
Since there is overlap between single and double groups, you will need to subtract, so:
Gym: 73 - 19 - 16 = 38
Pool: 59 - 19 - 9 = 31
Track: 31 - 9 - 16 = 6
Total for 1 facility: 38 + 31 + 6 - 4 = 71 people
((Minus 4 because the 4 triple facility users overlap the double facility users (problem is in layers: layer 1 minus layer 2, then minus layer 3).))
Next, add the totals:
71 + 40 = 111 people (using facilities)
135 - 111 = 24 people (who didn't use any)
(((I'm not 100% sure on this answer, so if someone could check my work, that would be much appreciated.)))
