Question 16864
The three equations you should use are:
A=5B (A is 5 times B)
C=B-16 (C is 16 less than B)
A+B+C=180 (Rule from geometry)

Substitue for A & C in terms of B in the third equation to get:
{{{(5B)+B+(B-16)=180}}}
You should be fine from here.