I think question is well worded as seniors are not be counted in adults ( Seniors and adults are taken as independent variables
Let A be the number of adult tickets sold, at $8 each
Let S be the number of senior tickets sold,at $7 each
Let C be the number of child tickets sold,at $5 each
So 8A +7S + 5C=1626 EQ 1
Now as the total tickets sold = 250
so, A+S +C= 250 Eq 2
C= A+30 Eq3
So Eq 2 Becomes
2A +S=250-30 =220 Eq 4
and Eq 1 becomes
13A+7S = 1476 Eq 5
Multiply Eq 4 by 7
Thus 14A +7S =1540 Eq 6
Solving Eq 5 and 6
A = 64, Adults
S=220-128 =92 , seniors
and C= 250-156= 94, Children