Question 947268
let y = your age
let s = sister's age
:
 I am twice as old as my sister was when I was the age that she is now.
y = 2(s - (y-s))
y = 2(s - y + s)
y = 2(2s - y)
y = 4s - 2y
y + 2y = 4s
3y = 4s
y = {{{4/3}}}s
 The sum of our present ages is 49.
y + s = 49 
replace y with {{{4/3}}}s
{{{4/3}}}s + s = 49
{{{7/3}}}s = 49
Multiply both sides by 3/7
s = 21 is sister's age
then
y + 21 = 49
y = 28 yrs is your age
:
:
See if that makes sense in the statement
the difference of the ages is 7 yrs
" I am twice as old as my sister was when I was the age that she is now."
28 = 2(21 - 7)
28 = 2(14)