Excel Help

I m a new to excel and I would like to know if there is a way to do the following

In cell M5 - Regardless of what one is selected from B5 to K5 , the value would display here *100%

In N5- Regardless of what 2 are chosen, they would be added together and multiplied by 90%

In O5 , Regardless of any 3 chosen , they would be added together and multiplied by 95%

So if I wanted any 7 communities added together and multiplied by a %age it would calculate in the cell automatically.