Question 10631
Let G = age of gym and A = age of auditorium.

G = A + 21 yrs.  The gym is 21 years older than the auditorium.
G = 3A The gym is 3 times as old as the auditorium.
Since G = G, then:
 A + 21 = 3A Solve for A.
2A = 21
A = 10.5 The auditorium is 10.5 years old.

G = 3A 
G = 3(10.5)
G = 31.5 The gym is 31.5 years old.