Total pressure of mixture of gases A, B and C = 10 atm.
Number of total moles = 10
Partial pressure of A = $\frac{No. of \, moles \, of \, A}{10} \times 10 $
$\therefore$ 3 = No. of moles of A
$\because$ Partial pressure of B = $\frac{No.of \, moles \, of \, B}{10} \times 10$
= No. of moles of B.
$\therefore$ No. of moles of C = 10 - (3 +1) = 6.
Now 1 mole of C = 2g
$\therefore$ 6 mol of C = 2 x 6 = 12 g