3. One drilling rig on an oil drilling rig can complete a job in 20 days. The second drilling rig would complete the same job in 30 days. If the first and second drilling rigs are joined by a third, all three together would finish the job in 7 days. How long would the third drilling rig take to complete the job on the drilling rig by itself?
3. The first drill completes $\frac{1}{20}$ of the job in 1 day.

The second drill completes $\frac{1}{30}$ of the job in 1 day.

The third drill completes $\frac{1}{x}$ of the job in 1 day.

All three drills together complete $\frac{1}{20}+\frac{1}{30}+\frac{1}{x}=\frac{1}{7}$ of the job in 1 day.

By multiplying the equation by the common denominator $420 \cdot x$ we get

$21 x+14 x+420=60 x$

$420=25 x$

$x=16.8$ days

1 POINT

$x=16$ days, 19 hours, and 12 minutes

The third drill would complete the job in exactly 16 days, 19 hours, and 12 minutes.