Bill ate 1/4 of a pound and then 1/6 of a pound.
All together, we can find out how much he ate if we add the fractions. But first, we have to expand them to a common denominator. We can see that a number that is both divisible by 6 and 4 is 12, so we expand until both of their denominators are 12.
1/4 * 3/3 = 3/12
1/6 * 2/2 = 2/12
Now add them, and you get 5/12. In total, Bill ate 5/12 of a pound of trail mix.