The sum of two digits of a two digit no is 15. If 9 is added to it, The digits are reversed.Find the number????

Hi!
Here is the answer to your question.
 
Let the tens digit be x and the ones digit be y.
x + y = 15                      … (1)
 
Original number = 10x + y
Reverse number = 10y + x
 
10y + x = 10x + y + 9  (Given)
⇒ 9y – 9x = 9
y – x = 1                        … (2)
Adding (1) and (2),
y = 8
When y = 8,
x + 8 = 15                ( from (1) )  
x = 15 – 8 = 7
∴ Original number = 10x + y = 10 × 7 + 8 = 70 + 8 = 78
 
Cheers!

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