According to question,
Ganga begins at 9 am and she does 3 units/hours
Saraswati begins at 10 am and she does 2 units/hours
So by 11 am they complete 5 units
Time $=\frac{\text{T}\text{.W}.}{3+2}=\frac{24}{5}$
(4 cycle of 2 hrs each + 4 units left)
And now ganga will complete 3 unit out of 4 units in 1 hr
Now, rest 1 unit work done by = $\frac{1}{2}$ hr
Total time = 8 + 1 + $\frac{1}{2}$ = 9$\frac{1}{2}$ hr
Hence,Work finished at
= 9 am + 9$\frac{1}{2}$ hr
= 6:30 PM

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Alternate Solution:
Work done by Ganga in 1 hour = $\frac{1}{8}$
Work done by Saraswati in 1 hour = $\frac{1}{12}$
They are working alternatively with Ganga beginning the job.
Work done in every two hours = $\frac{1}{8}$ + $\frac{1}{12}$ = $\frac{5}{24}$
Work done in 4 × 2 = 8 hours = $\frac{5\times 4}{24}$ = $\frac{5}{6}$
Remaining work = 1 - $\frac{5}{6}$ = $\frac{1}{6}$
In 9th hour, Ganga starts the work and does $\frac{1}{8}$ of the work
Work remaining = $\frac{1}{6}$ - $\frac{1}{8}$ = $\frac{1}{24}$
In 10th hour, Saraswati starts the work
Time needed to finish the remaining work
$\begin{array}{rl}& =\frac{\frac{1}{24}}{\frac{1}{12}}\\ & =\frac{1}{24}\times 12\end{array}$
  $=$ 0.5 hours
  $=$ 30 minutes
i.e., work will be completed in 9 hour 30 minutes, after 9 AM
i.e., at 6:30 PM