Answer:
sarah is 20 years old
Step-by-step explanation:
We will represent Mary as m, Sarah as s and wilfred as W
so m= s+3, w= 2m
and s + w + m = 89
substitute m for s+3
so
w=2s + 6 and
2s+3+w=89
substitute w for 2s+6
2s + 3 +2s+6=89
4s+9=89
4s= 80
s=20
