You'd be able to buy 16 salads.
First, you find the ratio for how many salads there are sandwiches:
15:20 = 3:4
Then you set up the ratio next to the current situation:
3:4 = 12:?
Then you're ready to figure it out, if four times three equals twelve sandwiches then we do the same to the number of salads, which is four times four which is...sixteen.