How to Solve Digit Problems Part I

Digit Problems is one of the word problems in Algebra. To be able to solve this problem, you must understand how our number system works. Our number system is called the decimal number system because the numbers in each place value is multiplied by powers of 10 (deci means 10). For instance, the number 284 has digits 2, 8, and 4 but has a value of 200 + 80 + 4. That is,

$(100 times 2) + (10 times 8) + (4 times 1) = 284$ .

As you can observe, when our number system is expanded, the hundreds digit is multiplied by 100, the tens digit is multiplied by 10, and the units digit (or the ones digit) is multiplied by 1. Then, all those numbers are added. The numbers 100, 10, and 1 are powers of 10: $10^2 = 100$ , $10^1 = 10$ , and $10^0 = 1$ . So, numbers with $h$ , $t$ , and $u$ as hundreds, tens, units digits respectively has value

$100h + 10t + u$ .

It is clear that this is also true for higher number of digits such as thousands, ten thousands, hundred thousands, and so on.

Many of the given numbers in this type of problem have their digits reversed. As we can see, if 10t + u is reversed, then it becomes $10u + t$ . For instance, $32 = 10(3) + 1(2)$ when reversed is $23 = 10(2)+ 1(3)$ . Now, that we have already learned the basics, we proceed to our sample problem.

Worked Example

The tens digit of a number is twice the units digit. If the digits are reversed, the new number is 18 less than the original. What are the numbers?

Solution and Explanation

The tens digit of a number is twice the unit digit. This means that if we let the units digit be $x$ , then the tens digit is $2x$ . As we have mentioned above, we multiply the tens digit with 10 and the units digit with 1. So, the number is

$(10)(2x) + x$ .

Now, when the digits are reversed, then x becomes the tens digit and $2x$ becomes the ones digit. So, the value of the number is

$(10)(x) + 2x$ .

From the problem above, the number with reversed digit is 18 less than the original number. That means, that if we subtract 18 from original number, it will equal the new number. That is,

$(10)(2x) + x - 18 = 10(x) + 2x$
$20x + x - 18 = 12x$
$21x - 18 = 12x$
$9x = 18$
$x = 2$
$2x = 4$