Question 987185
Hi there,
Matt is twice as old as his daughter Sally.
M = 2S
Ten years ago, the sum of their ages was 31.  
M - 10 + S - 10 = 31
Replace M with 2S from top equation
2S - 10 + S - 10 = 31
Collect like terms
3S = 31 + 20
3S = 51
S = 17
Matt is 34 years old
Sally is 17 years old
Hope this helps :-)