Çözüldü Proportion - Central Angle of a Circle

From a book's unsolved problems:

In a year a builder uses a total of 180 tons of sand, cement, and gravel, and he knows that he uses twice as much sand as gravel. If he represents this information on a pie chart and the sector indicating cement has an angle of 60°, how much sand does he use?

Cement = 180·60° / 360° = 30 tons
Gravel + Sand = 180 - 30 = 150 tons
30 + Gravel + 2·Gravel = 180 ⇒ Gravel = 50 tons
Sand = 2·Gravel = 2·50 = 100 tons.

Source:
"Exercises and Worked Samples in Statistics" by G.E. Skipworth, B TECH FSS AFIMA, Senior Lecturer in Statistics at Teesside Polytechnic, 2nd Edition, 1980, Page 2, Question 10, Answers: Page 132