60厘米 = 0.6米
0.6× 3.14× 50 ÷ 2000
= 94.2 ÷ 200
= 0.0471
= 4.71%少了一部？0.6× 3.14× 50 ×5 ÷ 2000= 94.2 ×5 ÷ 2000= 0.2355= 23.55%