3-digit numbers are formed using the digits 1,3,7 without...

Total 3 digits numbers formed by $1,3,7$ without repetition $=3 \times 2 \times 1=6$ Sum of digits $=1+3+7=11$ A number is divisible by 3 when the sum of digits of the number is either divisible by 3 or a multiple of 3 but 11 is neither the multiple of 3 nor divisible by 3 . $$\begin{aligned} & \therefore n(\mathrm{E})=0 & n(\mathrm{ S})=6 \\ & \text{ Probability }=\frac{n(\mathrm{E})}{n(S)}=\frac{0}{6}=0 \end{aligned}$$