For example a stands for the number of adult tickets, and s stands for the number of student tickets.
Write a system of equation based on the problem
An equation for "A total of 679 tickets were sold for the school play"
∴ a + s = 679 (first equation)
An equation for "there were 71 fewer student tickets sold than adult tickets"
∴ s = a - 71 (second equation)
Find the solution of the system
Subtitute s with a - 71 in the first equation to find the value of a
a + s = 679
a + a - 71 = 679
2a - 71 = 679
2a = 679 + 71
2a = 750
a = 750/2
a = 375
The number of adult tickets sold is 375