Lauren bought 20 ounces of ham. she used 3/4 of the ham to make sandwiches for her friends and 1/5 of the ham in an omlete. how many ounces of ham were left?

Lets divide the problem in parts, first for sandwiches she used 3/4 out of 20 ounces of ham:
(3/4)(20) = 60/4 = 15
so for sandwiches Lauren used 15 ounces
Then for omelete she used 1/5, that is:
(1/5)(20) = 20/5 = 4
then Lauren used 4 ounces for omelete
So Lauren used 15 ounces for sandwiches and 4 ounces for omelete, that is 19 ounces in total, so there are left from the 20 original ounces:
20 - 19 = 1
there is 1 ounce of ham left