It is a tricky question .Only hard part is understanding the word problem and then converting it into numbers. Let us concentrate on the problem now.
Let us assume the unknown number = x
The coming to the equation part, we get
6(2 - x) = 10
12 - 6x = 10
-6x = 10 - 12
-6x = -2
x = 2/6
= 1/3
So the unknown number is 1/3. I hope i have solved the question to your satisfaction and in future you can do this type of problems by yourself.