Lisa bought 5/8 of trail mix she give 1/8 pound of trail mix how much trail mix does lisa have left

Supposing that the trail mix was 5/8 pound, and Lisa gave 1/8 pound, then to know what is left we only have to subtract those fractions:

5/8 - 1/8 = 4/8

that is because we only subtract the numerators (upper part) of the fractions due to the fact that the denominators are equal, now, 4/8 can also simplify to 1/2

therefore there are left 4/8 pound of trail mix or, what is the same, 1/2 pound of it

Lisa has 4/8 or 1/2 trail mix left because all you had to do was do 5 - 1 which was 4, because the denominators are the same.