Question

The tens digit of a number is twice the ones digit. The sum of the digits in the number is 12. What is the number?

Answer 1

Let there be two variables: t and o where t is for ten digit and o is for one digit:

So we have two equations:
t = 2o (1)
t+o = 12 (2)

Subtract (2) from (1) and we get o = 12-2o

Now solve for o:

3o = 12

o = 4

Now plug in (1) or (2) and we have t = 8

Therefore our number is 10t + o = 84

Answer 2

Let x = the ones digit
Let y = the tens digit

x+2y=12
x=2y
x+y=12 substitute x for 2y to get:
(2y)+y=12 add like terms to get:
3y=12
3y=12 divide both sides by 3 to get:
y=4
x+y=12 substitute y for 4 to get:
x+4=12 subtract 4 from both sides to get:
x=8
so if x=ones
and y=tens
then the answer is 48