Three friends decide to rent an apartment and split the cost evenly. They each paid $640 towards the total move in cost of first and last month's rent and a security deposit. If rent is $650 per month, how much was the security deposit?

The three friends paid $640 each, so the total move-in cost was 3 × $640 = $1,920. First and last month's rent cost 2 × $650 = $1,300. Subtracting the rent from the total gives a security deposit of $1,920 − $1,300 = $620, which is choice C.

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