This would have to do with average. So if one does it in 6 days and the other does it in 4 days, you would do the number between them. So 5 is between 4 and 6.
The more complicated way of thinking of it is like this...
add 6 and 4 6+4 that would = 10
Because we added two numbers, we have to divide by two.
10/2 which equals 5.
If it was numbers closer together like. 6 and 7 the middle numbeR would be 6.5 for example.
