A math teacher saw a playground full of boys and dogs. He counted 30 heads and 100 legs, how many of them are boys?

Set up an equation and define some variables.

Let b be the number of boys.
Let d be the number of dogs.

Each dog and each boy has one head, and they add up to 30.

So, b + d = 30

Each boy has 2 legs and each dog has 4 legs, for a total of 100 legs (the teacher really counted all of them? :p)

This gives:

2b + 4d = 100

You have now two equations, and two unknowns. Can you now solve the problem? :)

Post what you get! :)

so 15 + 15 = 30

2(15) + 4(15) = 100

So there were 15 boys

Is this correct?

Not quite...

2(15) + 4(15) = 30 + 60 = 90

Try again :)

Do you know how to use substitution or elimination to solve simultaneous equations?

Somewhat, I've never been a big fan of these.

I will post my answer as soon.

Ok, I'll be waiting for you :)