Physics problem

Jack and Jill are the same age. Jack leaves earth on a spaceship at 0.5c. Jill waits one year then sends a radio message (message goes at speed of light) to Jack telling him to return. Jack upon receiving the message returns immediately to earth. How much younger is Jack than Jill?

gamma = 1/sqrt(1-0.5^2) = 2/sqrt(3)

Jack gets the message after he goes 1 light year, ie. after 2 years. He takes another 2 years to go back. So from his perspective 4/gamma = 3.46 years passed. Difference in age is 4 - 4/gamma = 0.54 years.