You and your friend decide to have a competition to see who can save the most for college. You open a new account and deposit $20/month.?

I imagine you're assuming this gains interest? If not then as others have stated its simply the amount you put in per month times the number of months and plus your initial deposit.

I'm assuming that the time, x, is months *after opening the account*.

You generally need to deposit money to open an account, so I'm assuming that $20 is deposited immediately, and $20 every month thereafter.

after 0 months, you have $20
after 1 month, you have $40
after 2 months, you have $60...

so y = $20 * (x+1)

An argument could be made for y = $20 * x

20x = y. what is so tough?