He sold one half of the pie because if you do it in your head split one half into 3 and get one sixth, then that slit in two is 1 twelfth so if he sold six pieces and six is one half of twelve then you get one half.
Its 30% because if you take 103 put it in the calculator and divide by 100 you get 1.03 and multiply that time 30 you get 30.9 thats how you find percent