Well, the first thing you would do is find like denominators for each fraction.
Since the smallest common term between the two is 40, that will be your denominator for each. Then you would multiply the numerator by the number your denominator was multiplied to get to 40. So, you would end up with 16/40 because 5*8=40 and 2*8=16 and 5/40 because 8*5=40 and 1*5=5.
After that, you would just subtract them.
16/40 - 5/40 = 11/40 and as a decimal that is 0.275. So, Joe has eaten 0.275 more pizza than Jane.
Hope this helps! :)