The Legend of Zelda: The Wind Waker -
Climb to the top of the island and you'll find Salvatore here. Speak to him and pay 50 rupees to play Salvatore's Barrel Shoot mini-game. In this game Link must lineup the cannon and change the angle in which it shoots. The trick here is to always uses degrees in multiples of '5' such as 15, 20, 25, 30, etc... Each time you play the game, the barrels are in different locations. After playing a few times, you should be able to recognize the actual distance of the canons.