You can ride your bike around your block twice and the whole neighborhood once in 15 minutes. You can ride your bike around your block twice and the whole neighborhood 5 times in 55 minutes. How long does it take you to ride around the neighborhood?

__ minutes

okay so lets make blocks = x and neighborhood = y

2x + y = 10

2x + 4y = 31

multiply the top equation by - 1 so we can single out y

-2x - y = - 10

2x + 4y = 31

combine the two equations

3y = 21

since the x's have canceled each other out, just divide both sides by 3 to get a single y

y = 7

it takes 7 minutes to go around the neighborhood. knowing that, you can also know it takes a minute and a half to go around the block.