Choose what the expressions below best represent within the context of the word problem.

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?

x represents digit
2x represents digit

Let's just try to solve the problem BA B is the tens digit A is the ones digit B = 2A A + B = 12 so by substitution for B A + 2A = 12 3A = 12 Can you solve for A ? Then solve for B and check your answer in the other equation 
I can see that the answer is 48... but let's work the problem Let the number be represented by xyy is the units digitx is the tens digit x+y=12.   the sum of the digits is 12y=2x.      the units digit is twice the tens digit replacing y with 2x in the first equation we have x+2x=123x=12x=4 y=2x =2(4)=8 xy = 48

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