What percentage of people order a salad or an appetizer to start?

Question

At a local restaurant, 55% of people order an appetizer to start while 25% order a salad to start. Also, 18% choose both an appetizer and a salad to start. What percentage of people order a salad or an appetizer to start?  

Answer 1

We have four possible orders here: (1) both and appetizer and a salad, (2) only an appetizer, (3) only a salad and (4) neither an appetizer nor a salad.

We know that 18% of diners fall into group (1), that 55% of diners fall into groups (1) or (2), and 25% of diners fall into groups (1) or (3).

If 18% of diners are in group (1), then 55% - 18% = 37% of diners are in group (2).

Similarly, with 18% of diners in group (1), 25% - 18% = 7% of diners are in group (3).

So 18% of diners are in group (1), 37% of diners are in group (2) and 7% of diners are in group (3), which means 18% + 37% + 7% = 62% of diners order a salad, an appetizer or both.

If you want the percentage who order just a salad or just an appetizer (but not both), that would be the percentage of diners in groups (2) or (3), which is 37% + 7% = 44%.