Let the two numbers be x and y 

\(\frac{1}{3}( x + y) = 12\)

then x + y = 36 ..........i

2( x - y) = 12

x - y = 6 ............ii

add equations i and ii 

2x = 42

x = 21, put x = 21 into equation i

x + y = 36

21 + y = 36 

y = 36 - 21 = 15

therefore the numbers are 21 and 15