a + a + a = 30
代表 30 ÷3 = 10 (a)
已知 a = 10,
( 20 - 10 ) ÷ 2 = 5 (b)
已知 b = 5,
( 10 - 5 ) ÷ 2 = 2.5 (c)
a = 10,b = 5,c = 2.5
10 + 5 + 2.5 = 17.5