Solution:
Let the two digits of a number be X and Y
Let the digit at 10's place = X
Let the digit at 1's place = Y
The two digit number = 10X+Y
Given that
The sum of the digits = 9
⇛ X+Y = 9
⇛X = 9-Y →→→→eqn(i)
If 9 is added to the number the digits interchanged
⇛10X+Y +9 = 10Y+X
⇛10X+Y+9-10Y-X = 0
⇛9X-9Y +9 = 0
⇛9(X-Y+1) = 0
⇛X-Y+1 = 0
⇛9-Y-Y+1 = 0
⇛10-2Y = 0
⇛2Y = 10
⇛Y = 10/2
⇛Y = 5
Now
X = 9-5 = 4
Therefore the number = 45 Ans.
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