Let the unit's digit be y and ten's digit, be x.

Then, the number = 10x + y

When interchang in the number is 10y + x.

According to the question,

\( \Large \left(10x + y\right) - \left(10y + x\right) = 18 \)

\( \Large 10x + y - 10y - x = 18 \)

= 9x - 9y = 18

= x - y = 2 ...(i)

and x + y = 12 ...(ii)

On adding Eqs. (i) and (ii), we get

x - y = 2

x + y = 12

2x = 14

x = 7

Therefore, x = 7 and y = 5

Product= \( \Large xy = 7 \times 5 = 35 \)