(I'm just using x and y because I'm used to those letters):

x + y = 30

Then we find out that one of the numbers is twice the other - x = 2y - we can then sub that into our first equation:

x + y = 30

2y + y = 30.........collect like terms

3y = 30...............divide by 3

y = 10

Now we know one of the numbers is 10, we can then use that and sub it into our second equation (x=2y):

x = 2y

x = 2(10)

x = 20

Therefore the two numbers are 10 and 20