The problem says that m∠TRS = 120º. The total number of degrees in a circle is 360º. 120º is a third of 360º. Therefore, we can prove that the shaded sector is a third of the circle.
The problem then says that the area of the shaded sector is 25π and we have to calculate the area of the entire circle. Since we already know that the shaded sector is a third of the circle, we can simply multiply 25π by 3 in order to calculate the area of t he entire circle.
