We know that: Standard deviation ( *.D.) = $sqrt("Variance")$
Var(X) = E(X2) [E(X)]2
E(X) = 2 0.2 + 3 0.5 + 4 0.3
= 0.4 + 1.5 + 1.2
= 3.1
E(X2) = 4 0.2 + 9 0.5 + 16 0.3
= 0.8 + 4.5 + 4.8
= 10.1
So V(X) = 10.1 (3.1)2
= 10.1 9.61
= 0.49
*.D. = $sqrt("Var"("X"))$
= $sqrt(0.49)$
= 0.7