Question 165451
let Adult =A and child=C     we have A+C=24   and we also know  that 9times A+3 times C=186 or 9A+3C=186.  Now  taking the first equation rewriting it to C=24-A and then substituting the value of A into the 2nd equation we get 
9A+3(24-A)=186 distributing the 2nd term we get 9A+72-3A=186 which simplifies to 6A=114  divide by 6 and A=19  C=24-A(19)=5

answer # of children is 5