Use 8/10 and 10/12. This represents how many spring rolls over how many minutes. Make your equations like this: 8/10 = 40/x and 10/12 = 40/x. Find x, which will give you the minutes it takes them to make 40 rolls.
You get x = 50 for the first one and x = 48 for the second. Nora makes 40 rolls in 50 minutes, and Eli makes 40 rolls in 48 minutes. Eli is faster.
All of the other answers share at least one factor they can be divided by (the numerator in all cases). 5/12 however shares no factors and cannot be simplified anymore.