Let x be the unknown number of years.
Let C be Chris' age and J be Jenny's age.
Then J+x = 2(C+x)-7
... describes this situation.
4+x = 2(2+x) - 7
4+x = 4 + 2x - 7
x = 2x -7 becomes 7=x
In 7 years, Jenny will be 7 years younger than twice Chris' age.
Check: Is this true? 4+7 = 2(2+7)-7
11 = 18-7, or 11=11 TRUE
