In a triangle, the second angle is twice the measure of the first, and the third angle is three times the measure of the second. find the measure of the third angle .
a. 20
b. 30
c. 120
d. 180

The solution to the problem is: 

               "The sum of the angles inside every triangle is 180 degrees."

The rest is just arithmetic.

First angle . . . . . A
Second angle = twice the first angle = 2A
Third angle = 3 times the second angle = 6A

The sum of the angles is  (A + 2A + 6A) = 9A

Since this is a triangle,
the sum is 180 degrees.      9A = 180

Divide each side by  9 :         A = 20

First angle . . . . . A = 20
Second angle = 2A = 40
Third angle = 6A = 120

Check: 
               Sum of the angles= (20+ 40 + 120) = 180        yay !