pls help our teacher didn’t explain how to solve these. also show work

Answer:
15) f = 18
3f = 54
6f = 108
16) k = 69
Step-by-step explanation:
15)
3f + 6f + f = 180
10f= 180
f = 18
16)
42 + k + k = 180
42 + 2k = 180
2k = 138
k = 69