Let J be the money that James saved and let S be the money of his sister.
We have that JAmes saved half the money that his sister saved, then:
then if they both save $30, then james will have saved 3/4 as much as his sister. To represent this in the equation, we have to add 30 to the previous eqution (since James saved $30):
But with the $30 more, James will have 3/4 as much as his sister:
Now, if we match both equations we get: