The sum of two numbers is 231. The larger is twice the smaller. What are the numbers?

Let x be the larger number.
Let y be the smaller number.

We're given two equations:

1. x + y = 231
2. x = 2y

Substitute (2) into (1) for x:
2y + y = 231
3y = 231

Type this into our math solver and get
y = 77

This means x is:
x = 2(77)
x = 154