.80 because you divide 100 by 5 and get 20 then multiply by 5 and get 100 so you multiply by 4 and get 80 but you move the decimal 2 places to the left.
Given the values of the three sides of the triangle, we can apply the Cosine Law to find the angles of the triangle. Recall that for we can express the value of c through the equation below.
Rearranging this equation, we can find the value ∠C as shown below.
We can apply the same reasoning for finding the value of ∠B as shown.
Plugging in the values of the sides (see image attached) from the given. It will now be straightforward to compute for ∠B and ∠C.