Answer :
the answer is D you get this from using simultaneous equations the working out is shown in the photo. ignore the stuff at the top of the page

Answer:
D. 13 children; 71 adults
Step-by-step explanation:
9a + 4c = 691
a + c = 84
9a + 9c = 756
- 9a + 4c = 691
5c = 65
c = 13
84 - 13 = 71
a = 71
Hope this helps!