The sum of two numbers is 30. the first number is twice the second number. what are the numbers?

Let the numbers be x and y.

by the first statement of the question, we get , x + y = 30.,,..(1)

by the second satement, we get , x = y+ y => x=2y... (2)

from equation (2), let us put the value of x in equation (1),
x + y = 30
=> 2y + y = 30
=> 3y = 30
=> y =30/3
=> y = 10

and x=2y => x = 2*10 = 20

in short, the numbers are 10 and 20