Hi there!
So there was a public pool opened for summer and a total of 246 people came swimming over the first 3 days. On the first day 79 came to swim and on the second day 104 came to swim. To find how much people swam on the third day, we need to add 104 + 79 and subtract the answer with 246.
104 + 79 = 183
246 - 183 = 63
63 people came to swim on the third day.
Hope this helped!~