From the question itself, it might be clear that because a minute is so much shorter than an hour (a minute is only 1/60th of an hour, because there are 60 minutes in each hour), 3/6 of a minute will be shorter than 4/6 of an hour.
But, if you wanted to calculate this the long way, you can.
To compare these two you can convert them to a unit of time that they have in common - for example, minutes.
In the first fraction, you can reduce 3/6 down to 1/2 - you probably already know that 1/2 is the same as half a minute.
Then you can multiply fractions to find out how many minutes are in 4/6 of an hour.
4/6 can also be reduced down to 2/3. You also know that there are 60 minutes in an hour.
(2/3 / 1 hr) x (1 hr/60 min) = (2/3) x 60 = 120/3 = 40 min
40 min is of course bigger than half a minute! So 3/6 of a minute is shorter.