Well, the greatest common factor of both 24, and 60 is 12. 24 ÷ 12 =2, while 60 ÷ 12 is 5, so the new fraction is 2/5.
The reason I divided the numerator and the denominator by the same number is because you are basically dividing by 1(12/12=1). Anything divided by 1 is itself.