w= weight of marks sister

Mark's weight = 74 lb. Mark's sister's weight = w lb. Since Mark and his sister together weigh 6lb. more than thrice the weight of Mark's sister alone, we can write: 74+w = 6+3w To solve this equation for w, first add -w to both sides of the equation.
74+w-w=6+3w-w or, 74 = 6+2w Now add -6 to both sides and simplify: 74-6 = 6+2w-6 or, 68=2w Therefore, w= (68/2) = 34 lb. Always verify if the answer is correct! Marks weight = 74 lb. Marks's sisters's weight = 34 lb. Sum of Mark and his sister's weight = 108
Three time Mark's sister's weight = 3*34 = 102 108 = 102+6 which is the staement of the problem. Final answer: w = 34 lb.