So this a filling time question
So we will use the simple mathematical equation
Let hot water takes t1 and cold water takes t2 time respectively
So t1= 15 min
t2= x min (let)
And let together they fill in 5 mins.
[tex]\frac{1}{t1}+\frac{1}{t2}=\frac{1}{t}[/tex]So by using above equation we can calculate this.
[tex]\begin{gathered} \frac{1}{15}+\frac{1}{x}=\frac{1}{5} \\ \frac{1}{x}=\frac{1}{5}-\frac{1}{15} \end{gathered}[/tex]Therefore x comes out to be 15/2 mins. i.e. 7.5 mins
So Cold water takes 7.5 mins to fill.