Sid had a collection of baseball cards. he gave half to his brother. one third to a friend. half of what he had left to another friend. now he has 50 left. what did he start with?

150. Honestly I did it in my head , but my thinking was 150 divided by 2 is 75. His brother got 75. He would have 75 left. Then, take 1/3 of 75 (25) and subtract it from 75. You then get 50, which is the amount he has left.