Question

a two digit number is such that the product of its digit is 20. If 9 is added to the number the digits interchange their places. find the number

Answer 1

let the digit in tens place be x and the digit in ones place be y
expanded form = y×10 + x×1
No. = 10y + x
10y×x = 20
yx = 20/10
yx = 2

If they interchange their places = 10×x + y
New no. = 10x+y

10y + x + 9 = 10x + y
10y = 10x + y - 9 - x
y = (10x + y -9 -x)/10

x = 10x + y - 9 - 10y

yx = 2

This you do on your own