Explanation
Let the unit place digit $$= x$$ Ten's place digit $$= 12 - x$$ Original number $$=$$ $$10\times(12 - x) + x$$ $$= 120 - 10x + x $$ $$=120 - 9x$$ Reversing the digit: Unit digit $$= 12 - x$$ Ten's digit $$= x$$ New number formed $$=$$ $$10\times x + 12 - x$$ $$= 9x + 12$$ As new number is less than the original number by $$18$$ New number $$=$$ original number $$- 18$$ $$9x + 12 = 120 - 9x - 18$$ $$18x = 120 - 30$$ $$18x =90$$ $$x = 5$$ Original number $$= 120 - 9x$$ $$= 120 - 9 \times 5$$ $$= 120 -45$$ $$= 75$$ Original number $$= 75.$$
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