1926 SAT horse and mules

Question

A dealer bought a lot of three horses

and some mules for $600, he sold the lot for 820.

If he gained $40 on each horse, and $50 for each mule,

how many mules were there?

Answer 1

The dealer made a profit of $220 ($820 - $600)

$120 of that profit came from the horses (3 x $40)

So $100 profit was made on the mules

 

How many mules was that?