Ganga and Saraswati, working separately can mow field in 8 and 12 hours respectively. If they work in stretches of one hour alternately. Ganga is beginning at 9 a.m., when will the moving be completed?

Explanation:-

1^st Method:
Whenever, workers are working alternatively on one work, we take 2 as 1 unit
In this case, we take 2 hours as 1 unit.
Part of the field moved by Ganga and Saraswati in 2 hours (1 unit) = $$\frac{1}{8}$$ + $$\frac{1}{{12}}$$ = $$\frac{5}{{24}}$$
Time taken to complete the work,
= $$\frac{1}{{\frac{5}{{24}}}}$$ = $$\frac{{24}}{5}$$ unit of time
Then actual time taken by them to complete the work,
= $$2 \times \frac{{24}}{5}$$   = 9.6 hours
The work starts at 9 a.m. then it will complete at 6:36 pm

2^nd Method:
% of the field moved by Ganga and Saraswati in 2 hours (1 unit),
= $$\left( {\frac{{100}}{8}} \right)\%$$   + $$\left( {\frac{{100}}{{12}}} \right)\%$$
= 12.5 + 8.33
= 20.83%; Time taken to move the whole field,
= $$\frac{{100\% }}{{20.83\% }}$$
= 4.8 unit of time;
Hence, actual time = 2 × 4.8 = 9.6 hours.