Question

A number has two digits whose sum is 9. If 27 is added to the A
number, its digits get interchanged. Find the number.
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Answer 1

$10x+y$ - a number

$x+y=9\\10x+y+27=10y+x\\\\x+y=9\\9x-9y=-27\\\\x+y=9\\\underline{x-y=-3}\\2x=6\\x=3\\\\3+y=9\\y=6\\\\10\cdot3+6=36$

So, it's 36.