If a two digit positive number is multiplied by its unit digit, then the product is 189 and if the tens' digit is twice of the unit digit, then calcutate the unit digit.

Explanation

Let the unit digit =x, `:.` tens's digit =2x.
`:.` the number `=10xx2x+x=20+x=21x.`
As per question, `21x"xxx=189,or,21x^(2)=189`
or, `x^(2)=(189)/(21)orx^(2)=9or,x=pm3`.
But the value of x can not be negative, `:.x=3`.
Hence the required unit digit =3.