Hi ! so I'm not sure at all by at least I tried.
First of all, you know that :
the first tap fill the pond in one hour, which is 1h/pond
the second tap is then 2h/pond
the third one is 3h/pond
and finally the fourth is 4h/pond.
Now let's see in one hour, how many ponds can we fill :
the first 1h/pond = 1 pond
the second 2h/pond = in one hour 0,5 pond
the third 3h/pond = in one hour 0,333333 pond
the fourth 4h/pond = in one hour 0,25 pond
then, if every tap is opened, then in one hour we can fill 0,25 + 0,333 + 0,5 + 1 = 2,08 ponds aproximatively, which is too much.
We can now use a proportionality table as:
1 hour = 2,08 ponds
x = 1 pond
x = 1*1/2,08 = 0,48 (still approximatively)
but this answer is not in seconds, to do this you just have to :
0,48*60 = 29 minutes.
To conclude, 29 minutes are needed to fill this pond.
