Number of cards of John: J
Number of cards of Alan: A
John and Alan have a collection of x baseball cards:
J+A=x (1)
John has x/4 cards:
J=x/4
What fraction of the cards does Alan have?
Replacing J by x/4 in the equation (1)
(1) J+A=x
x/4+A=x
Solving for A:
x/4+A-x/4=x-x/4
A=4x/4-x/4
A=(4x-x)/4
A=3x/4
Answer: Alan has 3/4 of the cards