The total admissions (T) is equal to the number of adult admissions (A) plus the number of child admissions (C) so if:
A = # of Adult Admissions and
C = # of Child Admissions and
1900 = Total # of Admissions then
A + C = 1900
Also the total paid for adult Admissions ( Pa = 13 * A) plus the total paid for child admissions (Pc = 7 * C) is equal to the total admission collected (21,700).
13A + 7C = 21,700.
Therefore you can solve the system of equations for A & C and the answer is A.
A + C = 1900
13A + 7C = 21,700
Use ellimination method by adding -7 times the first equation to the 2nd equation to get
6A = 8400
Divide both sides by 6 and get
A = 1400.
so 1400 adult admissions were sold.