Algebramedium · Past Paper
A two-digit number is such that the sum of its digits is 9. If the digits are interchanged, the number increases by 9. Find the number.
A45
B54
C36
D63
✓ Correct Answer: A — 45
Let number be 10x+y. x+y=9. (10y+x) - (10x+y) = 9 -> 9y-9x=9 -> y-x=1. Solving x+y=9 and y-x=1 gives y=5, x=4. Number is 45.
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