there are 60 minutes in 1 hr, so the ratio could be written as 60m/hr OR hr/60m, one atop or at the bottom, it really depends on which one you need to cancel out.
and there are 60 seconds in 1min, so 60s/min or min/60s.
now, we have 3.4 hours, we want to cancel the hours, so we'll use the ratio with the hours at the bottom, and we also would like to cancel out the minutes, so we'll use the ratio that has the minutes in the opposite location, what the heck does all that mean? let's see it,
[tex]\bf 3.4\underline{hr}\cdot \cfrac{60\underline{min}}{\underline{hr}}\cdot \cfrac{60sec}{\underline{min}}\implies \cfrac{3.4\cdot 60\cdot 60~sec}{1}\implies 12240~sec[/tex]
notice, the minutes are atop, so we use the ratio with it at the bottom to hose it, and all we're left with, is the seconds unit.
