In many question you need to use what people call "croo-multiplying".

Here is what we know
1,200milimeters ------>1h
In the question, they ask us an answer in seconds, so let's convert our figures.
1,200milimeters ------> 3600seconds
1 milimeter ------> X seconds

Now you can cross multiply the above datas:
3600*1 = 1,200*X
3600/1200=X
X=1/3




If the question would have been, number of milimeters which leak in 45 seconds, you write that:
1,200milimeters ------> 3600seconds
X milimeter ------> 45 seconds
And you cross-multiply in the same way to find X

hi pepe,

isnt the answer 3 and not 1/3??

we know that it leaks 1200 ml/h
or 1200ml/h / 60 min/h = 20 ml/min
or 20 ml/min /60 sec/min = 1/3 ml/sec

1 ml / 1/3 ml/sec = 3 seconds

No need for cross multiplication... just some division and unit checking.

Also note that you can skip a couple steps:

we know it leaks at 1200ml /3600 seconds
We want to find the how long it takes to drain 1ml... which is really just asking us to find x in:
1 ml / x seconds
So really, all you need to do in this question is divide 3600 by 1200 and get 3.

That is 3
My error is between the two last lines
3600/1200=X
X=3 and not 1/3

And it's logically impossible.