Sum and average of Monday and Wednesday only?

I have an attendance spread sheet for subjects:

 

Day      Monday  Tuesday  Wednesday  Thursday

Bio         11           4                5             9

Chem     9             7                 6           10

Art        13            0                  4           1

 

I want a formula that gives me the sum for only Monday and Wednesday Biology attendance without calculating Tuesday and Thursday.

 

 

 

By: Khaled Gamal

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