:why does the aa similarity theorem only need to identify the measurement of 2 congruent angles? why is it not necessary to find the measurement of the third angle and show that it is also congruent to the corresponding third angle of a similar triangle?

According to the third angle theorem states if two angles are congruent to two angles in another triangle, the third angles are congruent too. Because a triangle has 180 degrees the third angle in any triangle is 180 degrees  minus the other two angle measures

There is 240 red cars and there is 400 blue cars because 240/3=80 and 80*5=400