Question 351456
Hi,
*Note: Mark was and "always" will be 4 yerars younger than his sister.
Let x be his sister's age 6 years ago. His age at that timet would have been (x-4)
.
Question states:
x = 5*(x-4)
.
simplify and solve
x = 5x - 20
20= 4x
x = 5 (his sister's age 6 years ago)
. 
Now: his sister is 11 and he is 7 (4 years younger)