4 months because:
1 month = 5 dollars
so you add 5 until you get to 20 dollars. And as many times as you added 5 is how
many months. So:
5= 5
5 + 5 = 10
5+5+5= 15
5+5+5+5= 20
^ ^ ^ ^
1 2 3 4
your answer is 4 months ^^
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