Mathematics - Permutation & Combination Question with Solution | TestHub

Let the number be $\begin{array}{rl}& 2ab,a+b=13\\ & \Rightarrow a,b\in \{0,9\}\\ & \Rightarrow 6\text{ numbers }\{(9,4),(8,5)\dots (4,9)\}\end{array}$ Similarly, for $3ab,a+b=12\Rightarrow 7$ numbers For $4ab,a+b=11\Rightarrow$ Numbers For $5ab,a+b=10\Rightarrow 9$ numbers For $6ab,a+b=9\Rightarrow 10$ numbers For $7ab,a+b=8\Rightarrow 9$ numbers For $8ab,a+b=7\Rightarrow 8$ numbers For $9ab,a+b=6\Rightarrow 7$ numbers $\therefore$ Total ways $=64$.