TT -
Mr. Holms used 4/5 of the carton already, right? So that means he has 1/5 of the carton remaining.
He used half of that 1/5 to make 2 servings. 1/5 divided by 2 is just 1/5 times 1/2 which is 1/10.
Mr. Holms used 1/10 of the carton for each serving.
Check Your Answer: 4/5 + 1/10 + 1/10 = 8/10 + 1/10 + 1/10 = 10/10 = 1 whole carton. BOMB! Right on the money.