If a problem says one number is twice as large as another, which number should I choose as my variable?
I suggest that you use concrete numbers at first to set up the equation or expression. Once you are able to see how the numbers are put into place then you can substitute out the numbers for variables where necessary.
In this case you should make the variable the smaller number.
Lets say that the first number is a
If you decided to use variable we would say that
So your expressions would be