Tractors

Six tractors started plowing the field, and work together took 12 days.

Agronomist hesitates to

A) after two days to withdraw two tractors, or
B) after three days, withdraw three tractors.

Help him which of these two cases will be plowing done sooner.

Result

A

B

Step-by-step explanation:

$$\begin{gathered} v=\frac{1}{6\cdot 12}=\frac{1}{72} \\ 6\cdot 2\cdot \frac{1}{72}+4\cdot A\cdot \frac{1}{72}=1 \\ 12+4A=72 \\ A=60/4=15d \\ 6\cdot 3\cdot \frac{1}{72}+3\cdot B\cdot \frac{1}{72}=1 \\ 18+3B=72 \\ B=54/3=18d \\ A<B \end{gathered}$$

Try another example