Lucy is k years old so we know k is an important part of this problem. Sister 1 is 3 years younger, so she is k - 3 years old. Sister 2 is 5 years less than twice Lucy's age therefore, she is 2k (twice Lucy's age) - 5 years old. Now we have: k + (k - 3) + (2k - 5) We can simplify like terms with the k's and get 4k. then we add (-3) and (-5) to get 4k - 8.