Let's use some letters to stand for each boy.
T = Trevor
J = Jim
A = Alex
If Trevor has four times the number of baseball cards that Jim has, then we can write 4J for Trevor instead of T (T = 4J).
If Alex has half the number of cards that Trevor has, then A = 2J (since 2 is one-half of 4).
SO:
J + 4J + 2J = 35
7J = 35
J = 5
Therefore, Jim has 5 baseball cards. Trevor has four times that many, or 20 cards, and Alex has half as many as Trevor, or 10 cards.
Let's check to see if our answer is right:
5 + 4(5) + 2(5) = 35
5 + 20 + 10 = 35