CAT 2018 Question Paper Shift 2 Solved Paper

Let 'ab' be the two digit number. Where $\mathit{b}\ne 0$.
On interchanging the digits, the new number will be 'ba'
As per the condition $10\mathit{a}+\mathit{b}>3\times \left(10\mathit{b}+\mathit{a}\right)$
$7\mathit{a}>29\mathit{b}$
For $\mathit{b}=1,\mathit{a}=\left\{5,6,7,8,9\right\}$
For $\mathit{b}=2,\mathit{a}=\left\{9\right\}$
For $\mathit{b}=3$, no value of 'a' is possible.
Hence, there are a total of 6 such numbers