Question

the sum of the digit of two number is 7 if the digits are reversed the number formed is 9 less than the original number find the number

Answer 1

Let the digits be x and y .
Number will be 10x+y.
As per the question,
10x+y -9 = 10y+x.
9(x-y) = 9
x-y= 1.
And, x+y = 7 is given
By solving both the equations,
you will get
x= 4, y = 3
Hence, Original number will be 43.

Answer 2

let the tens digits be X and ones digit be y
X + y = 7
the number = 10X+y
after the digits are reversed the number formed = 10y+X
now
10y+X = 10X+y -9
9X-9y = 9
9(X-y)=9
X-y = 1
now
X+y = 7
X-y = 1
X = 4
y = 3
therefore
number = 4*10+3 = 43