Question 572769
I understand everything except the last sentence.  I'm going to proceed anyway and hope this helps you.


Sally's age now = S
Peter's age now = P

Sally's age 5 years from now = S + 5
Peter's age 5 years from now = P + 5


We also know a few facts:
S = P + 40  Sally is 40 years older than Peter
(S+5) = 2(P+5)


Let's use the first equation to substitute for Sally's age (S) in the second equation:
((P+40) + 5) = 2(P+5) Since the left side is all addition, we can remove the parens without a problem:
P + 40 + 5 = 2(P + 5)
P + 45 = 2(P + 5)  Now multiply out the paren on the right:
P + 45 = 2P + 10  Subtract P from both sides:
45 = P + 10   Subtract 10 from both sides:
35 = P


Peter is 35.  Since Sally is 40 years older than Peter, she is 75.


Checking our answer:
In 5 years, she'll be twice his age:
Peter will be 35 + 5 = 40 in 5 years.  
Sally will be 75 + 5 = 80 in 5 years
Sally will be twice Peter's age.  Correct!