30 M&Ms
1) Gathering the data
12-ounce bag of trail mix = 18 M&Ms
20-ounce bag=?
2) Assuming that the M&Ms are placed in following a constant rate we can write the following proportion:
12 --------------- 18
20 --------------- y
Cross multiplying it due to the first property of proportions
12y =20 x 18
12y = 360
y=30
3) So, I'd expect to find 30 M&Ms at a 20-ounce bag of trail mix.