It is given that
Weight of wheat $$= 3 \dfrac{3}{4} kg = 15/4 kg$$
Weight of rice $$= 2 \dfrac{1}{2} kg = 5/2 kg$$
So the total weight of wheat and rice $$= 15/4 + 5/2$$
Here the LCM of 4 and 2 is 4
$$= \left(15 \times 1 \right)/\left(4 \times 1 \right) + \left(5 \times 2 \right)/\left(2 \times 2 \right)$$
So we get
$$= \left(15 + 10 \right)/4$$
$$= 25/4 kg$$
$$= 6 \dfrac{1}{4} kg$$