f + s + t = 84 (1)
s = 3t (2)
t = f + 6 (3)
substitute for s in (1)
f + 4t = 84 (4)
substitute for t in (4)
f + 4(f + 6) = 84
simplify
5f + 24 = 84
f = 12
sub for f in (3)
t = 18
sub for t in (2)
s = 54
Check
f + s + t = 84 - answer is correct.
The sum of three numbers is 84 . The second number is 3 times the third. The third number is 6 more than the first . What are the numbers?