If 18 persons can build a wall 140 m long in 42 days, the number of days that 30 persons will take to complete a similar wall 100 m long, is.	Choose the correct choice
A) 18
B) 21
C) 24
D) 28
E) 30
Explanation :   (Length 140:100) : (Persons 30:18) :: 42 : x   =&gt; 140 * 30 * x = 100 * 18 * 42   =&gt; or x = 18   18nswer : 18
<result>A</result>
If 18 persons can build a wall 140 m long in 42 days, the number of days that 30 persons will take to complete a similar wall 100 m long, is.