Mathematics - Probability Question with Solution | TestHub

If $a\ne 0$ the numbers with $O$ \& a are $aooo$, aooa, aoao, aaoo, aaao, aaoa \& aoaa. These are $9\times 7-63(a+b,ab+0)$ Now, aaab, aaba, abaa, baaa are $4\times 9\times 8=288$ \& aabb, abab, abba are $3\times 9\times 8=216$ \& aaaa 9 $\text{ ∴ prob }=\frac{288+216+9}{63+288+215+9}=\frac{513}{576}=\frac{57}{64}$