Question

This is a popular type of problem that appeared in mathematics textbooks in the 1970s and 1980s. can you find the answer? the sum of the digits of a two-digit number is 6. if the digits are reversed, the difference between the new number and the original number is 18. find the original number.

Answer 1

A digit is a number in one of the places, so for example the number 54 has two digits; a tens place digit (5) and a ones place digit (4).

Say the mystery number is a two digit number = xy
* that's not x times y but two side by side digits.

Info given:
the sum of the digits of a two-digit number is 6
x + y = 6
if the digits are reversed, yx the difference between the new number and the original number is 18.

**To obtain the number from digits you must multiply by the place and add the digits up. (Example: 54 = 10(5) + 1(4))

Original number = 10x + y
Reversed/New number = 10y + x

Difference:
10y + x - (10x + y) = 18
9y - 9x = 18
9(y - x) = 18
y - x = 18/9
y - x = 2

Now we have two equations in two variables
y - x = 2
x + y = 6

Re-write one in terms of one variable for substitution.
y = 2 + x
sub in to the other equation to combine them.
x + (2 + x) = 6
2x + 2 = 6
2x = 6 - 2
2x = 4
x = 2

That's the tens digit for the original number. Plug this value into either of the equations to obtain y, the ones digit.

2 + y = 6
y = 4

number "xy" = 24