It is given that
Weight of Vijay $$=65 \dfrac{1}{2}$$kg
Weight gained during first week $$=1 \dfrac{2}{5}$$kg
Weight gained during second week $$=1 \dfrac{1}{4}$$kg
Weight lost during third week $$= 5/16$$ kg
So the weight of Vijay after third week $$= 65 \dfrac{1}{2} + 1 \dfrac{2}{5} + 1 \dfrac{1}{4} - 5/16$$
It can written as
$$= 131/2 + 7/5 + 5/4 - 5/16$$
LCM of $$2, 5, 4$$ and $$16$$ is $$8$$
$$= \left (5240 + 112+ 100 - 25 \right)/80$$
By further calculation
$$= \left (5452 - 25 \right)/80$$
$$= 5427/80$$ kg
$$= 67 \dfrac{67}{80}$$ kg