Start by creating an equation. You’ll need a variable to represent what is earned per week, for example x.
So,
420=60+9x
(Since he already has 60)
Subtract the 60 from 420 to begin isolating the variable
This gives you
360=9x
Divide both sides by 9 to completely isolate the variable
This gives you
40=x
So, he needs to save up $40 each week