There's a "3 for 2" (or "2+1" if you like) offer on mangoes. For a given quantity and price (per mango), calculate the total cost of the mangoes.
mango(2, 3) ==> 6 # 2 mangoes for $3 per unit = $6; no mango for free
mango(3, 3) ==> 6 # 2 mangoes for $3 per unit = $6; +1 mango for free
mango(5, 3) ==> 12 # 4 mangoes for $3 per unit = $12; +1 mango for free
mango(9, 5) ==> 30 # 6 mangoes for $5 per unit = $30; +3 mangoes for free