If you are in the lower grades:

The sum of the digits of a 2 digit number is 6. 

So it's either 15, 24, 33, 42, or 51.

The second digit is twice the first digit. 

So it's 24.

If you are in the upper grades,

t = the tens digit
u = the units digit

The system to solve is:



Substitute 2t for u in the first equation:

t+2t = 6
  3t = 6

Divide both sides by 3

  = 

  = 

  = 2

u = 2t = 2(2) = 4

u = 4

The number = 24

Edwin