Answer :
We will have the following:
First, we determine the rate of filling for each pipe as follows:
[tex]\begin{cases}p_1=\frac{6000l}{75\min}\Rightarrow p_1=\frac{80l}{\min} \\ \\ p_2=\frac{6000l}{50\min}\Rightarrow p_2=\frac{120l}{\min }\end{cases}[/tex]So, the first pipe fills it at a rate of 80 liters per minute and the second one at a rate of 120 liters per minute.
Now, we determine the time it would take to fill it for the two rates combined, that is:
[tex]p_3=p_1+p_2\Rightarrow p_3=\frac{200l}{\min }[/tex]So:
[tex]x=\frac{6000l\cdot1\min}{200l}\Rightarrow x=30\min [/tex]So, it will take 30 minutes to be filled by both pipes at the same time.