Let's use "x" as the equal amount of money.
Jane now has 7+x dollars
Her sister had 2+x dollars
Since Jane has twice the amount of her sister's money, her sister's money now becomes 2(2+x), or 2 times 2+x
Now just solve for x.
7+x = 2(2+x)
Distribute the 2 to 2+x
7+x = 4 + 2x
Subtract x from both sides
7 = 4 + x
Subtract 4 from both sides
3 = x
Their parents gave them $3.
If you want to check your work, just plug in 3 as x for their equations.
Jane: 7 + 3 = 10
Jane's sister: 2 + 3 = 5
10 is in fact twice of 5.