Pre BPI test help

I am confused on how they are getting the answer on a pre BPI test and its a simple question and I would like help with the formula they use and how they acquire the answer:

A 200 square foot wall with a R-value of 12 has a 30 square foot window with a U factor of 0.8 what is the approximate weighted R value of this wall?

Answer was 5.3

This may help you understand.

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Thank you and when I use this method It seems to come up wrong. this is where I got the pre test from is this site. I will keep trying.