Organisms that reproduce sexually have greater genetic and thus characteristic variations, and so, are able to withstand more selection pressures if they so happen to change or become greater. This increases the chances of survival for that species itself.
Organisms that reproduce asexually are genetically identical to each other. They are unlikely to withstand any changes in selection pressures.
To get an average, you must add all of the numbers together, and divide that sum by the amount of numbers there are. There are 6 number, which is what we must divide by.