A: the first answer is the best option
if there is a total of 5000 tickets, and we know there were adults, children, and seniors, then the equation:
c + a + s = 5000 is correct
if we are using c, a, and s as variables for how many children, adults, and seniors were in attendance, then by matching the corresponding price, we should have the equation:
$72000= 10c + 20a + 15s
lastly, if we know the amnt of children in attendance was 3x more than the amnt of seniors, the equation:
3s = c is best because by multiplying the number of seniors in attendance by 3, you will get 3x more children than seniors
hope this helps, and is correct :)!