Solve this if you can

30 men 7 hrs need 18 days, 30 men 6 hrs will need 18x7/6=21 days , 1 man 6 hr will need 21x30 days, Therefore to finish the work in 30 days we need 21x30/30 men=21 men

Let $x$ be the required number of laborers, then

$x\cdot 6 \cdot 30 = 30 \cdot 7\cdot 18$

$x=21$

30 * 7 * 18 = 3780
L * 6 * 30 = 3780
L = 3780/180 = 21 (Ans.)

suppose each worker does 1 joule of work each hour....... so the amount of work to be done to complete the job is ( 30 * 7 * 18) joules............ now if X laborers are employed for 6 hrs a day and the work is to be completed in 30 days ...... the work to be done will be ( 30 * 6 * X )
now equate .............. ( 30 * 7 * 18 = 30 * 6 * X ) ........... which gives x= 21.

let laborers be x : 30 laborers 7 hrs 18 days= 30 7 18 = 3780 ......x laborers 6 hrs 30 days= x 6 30 = 3780.........
x=3780\180 =21.....................21

$\frac{1}{n}=\frac{1}{30\cdot 7\cdot 18} \cdot 6 \cdot 30 = \frac{1}{21}$