Question 1206440
<pre>
As the tutors fight each other for egos, I'll reword the question:

A farmer ploughs his land in 12 days if he uses 5 tractors (with operators)
how long will it take if he uses only 3 tractors (with operators).

They've already solved it, but let's use the "worker-time-job" formula,
which even takes into account the number of jobs required to be done.
In this case the number of jobs to be done in both situations is 1, so
J<sub>1</sub> = J<sub>2</sub> = 1. 

{{{(W[1]T[1])/J[1]}}}{{{""=""}}}{{{(W[2]T[2])/J[2]}}}

where

W<sub>1</sub> = the number of workers in the first situation.
T<sub>1</sub> = the number of time units (days in this problem) in the first situation.
J<sub>1</sub> = the number of jobs in the first situation.

W<sub>2</sub> = the number of workers in the second situation.
T<sub>2</sub> = the number of time units (days) in the second situation.
J<sub>2</sub> = the number of jobs in the second situation.

W<sub>1</sub> =  5             W<sub>2</sub> = 3     
T<sub>1</sub> = 12             T<sub>2</sub> = the unknown quantity 
J<sub>1</sub> = 1              J<sub>2</sub> = 1

{{{(W[1]T[1])/J[1]}}}{{{""=""}}}{{{(W[2]T[2])/J[2]}}}

{{{(5*12^"")/1^""}}}{{{""=""}}}{{{(3*T[2])/1^""}}}

{{{60}}}{{{""=""}}}{{{3T[2]}}}

{{{20}}}{{{""=""}}}{{{T[2]}}}

Answer: 20 days. 

Edwin</pre>