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The ten's digit of the two-digit number is two times the unit digit and the difference between the original number and the interchanged number is 18. What is the original number?
1. 36
2. 63
3. 24
4. 42

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Correct Answer - Option 4 : 42

Concept used:

Number = 10(Tens digit) + Unit digit

Ex:- 50 = 10 × 5 + 0 = 50

Calculation:

Let the unit and ten's digit of the number be x and 2x respectively

Number = 10(2x) + x = 21x

Interchanged number = 10x + 2x = 12x

Now, A/Q

21x - 12x = 18

⇒ 9x = 18

⇒ x = 2

Number = 21x = 21 × 2 = 42

∴ The required number is 42

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