let the price of the senior citizen tickets be s while that if the child tickets be c
As such, if on the first day of ticket sales the school sold 2 senior citizen tickets and 11 child tickets for a total of $155. Then
2s + 11c = 155
where on the second day, the school took in $142 on the second day by selling 2 senior citizen tickets and 10 child tickets then
2s + 10c = 142
solving both simultaneously, equation 1 minus 2
11c - 10c = 155 - 142
c = 13
sinec 2s + 11c = 155
2s + 11(13) = 155
2s + 143 = 155
subtract 143 from both sides
2s = 155 - 143
2s = 12
divide both sides by 2
s = 6
The price of one senior citizen ticket is $6 and that of the child is $13.