Find the sum of the interior angle measures of a regular polygon with 20 sides

Afactory buys 10% of its components from suppliers b and the rest from supplier c. it is known that 6% of the components it buys are faulty. of the components brought from suppliers a,9% are faulty and of the components bought from suppliers b, 3% are faulty. find the percentage of components bought from supplier c that are faulty.

How is this equation completed? i cannot find any examples in the book.

Find the solution to this system 2,1 1,-2 1,2 -1,2