(a)
$SO_{2} + \ \ \ \ \ \frac{1}{2}O_{2}\ \ \ \ \rightleftharpoons {SO}_{3}$
$5\ mol\ \ \ \ \ \ 5\ mol\ \ \ \ \ \ \ \ \ \ \ 0\$ initially
$(5 - x)\left( 5 - \frac{1}{2}x \right)\ x$ at equilibrium
$x = \frac{60}{100} \times 5 = 3$
Total number of moles $= (5 - x) + \left( 5 - \frac{1}{2}x \right) + x$
$= (5 - 3) + \left( 5 - \frac{1}{2} \times 3 \right) + 3$
$= 8.5$