Question

In a decimal number, the ones digit of a number is twice the digit at the tenths place. On interchanging the digits of the ones place with the digit at the tenths place, we get a number which is 1.8 less than the original number. What is the decimal number?

• A. 4.2
• B. 2.1
• C. 8.4
• D. 6.3

Answer 1

The correct option is A 4.2

Let the tenths place digit is $x$. Then, the ones place digit is $2x$. So the number is actually
$2x+0.1x=2.1x$

After interchanging the digits we get a number,
$x+0.1\times 2x=1.2x$.

According to the question, the interchanged number is 1.8 less than the first number.
$\therefore 2.1x-1.2x=1.80.9x=1.8x=\frac{1.8}{0.9}x=2$

Hence, the tenths place digit is 2 and the ones place digit is $2x=4.$ So, the number is 4.2.