In a crystalline solid, atoms of element P form a ccp lattice....

Let the number of atoms of element P in the ccp lattice be $N$.

The number of tetrahedral voids is $2N$ and the number of octahedral voids is $N$.

Number of Q atoms = $\dfrac{1}{3} \times 2N = \dfrac{2N}{3}$

Number of R atoms = $\dfrac{1}{2} \times N = \dfrac{N}{2}$

The ratio of the number of atoms of P, Q, and R is:
P : Q : R = $N : \dfrac{2N}{3} : \dfrac{N}{2} = 1 : \dfrac{2}{3} : \dfrac{1}{2}$

Multiplying by $6$ to get whole numbers, the ratio is $6 : 4 : 3$.

The empirical formula of the compound is $P_{6}Q_{4}R_{3}$.

Molar mass of the empirical formula = $(6 \times 20) + (4 \times 40) + (3 \times 40)$
$= 120 + 160 + 120 = 400\text{ g mol}^{-1}$

Mass percentage of P = $\dfrac{\text{Mass of P}}{\text{Total mass}} \times 100$
$= \dfrac{120}{400} \times 100 = 30\%$

Answer: $30$