W-a C/S ratio

Thank you Sir for your previous answers.
I’m confused with problem on multi product.
In one of problems, WA CS ratio is calculated as adding all the contribution and then dividing over number of products, the Revenue is calculated in the same way.
In the other problem Contrib. and Sales are arrived at by just adding the figures but not divided over number of products.
Which way is correct?
Let me depict it as following:
1)((Product 1 Contrib. + Product 2 Contrib. + Product 3 Contrib. )/3 )/((Product 1 SR+ Product 2 SR + Product 3 SR)/3))

2)
1)(Product 1 Contrib. + Product 2 Contrib. + Product 3 Contrib. )/(Product 1 SR+ Product 2 SR + Product 3 SR)