The guitar costs $100 every month for 12 months so it costs 100*12 = $<<100*12=1200>>1200
His dad is charging him 10% interest on the $1200 so interest comes to .10*1200 = $<<1200*.10=120.00>>120.00
All total, Aaron will owe his father the price of the guitar, $1200 and the interest, $120 so 1200+120 = $<<1200+120=1320.00>>1,320.00
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