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