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?

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

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

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