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.

We'll write the equation equivalent to the statement:
The sum of the numbers is 30.

x + y =
30

Let the 1st number be
x.

Since the 1st number is twice the second, we'll re-write
the equation above:

2y + y =
30

3y = 30

y =
10

x = 30 - y => x =
30-10

x = 20

The
requested numbers are x = 20 and y = 10.