Two students on bicycles leave their classroom building at 10: 00 a. m. and travel in opposite directions. if the average speed of one of the students is 12 km/h and the average speed of the other one is 14 km/h, at what time will they be 65 kilometers apart?

Now... both are traveling in opposite direction
after some time "t", they cover altogether a distance of 65 miles
let's say, the one at 12mph cover a distance of "d",
thus
the other biker, cover the difference of that and 65, or 65-d
whatever "d" is

now, the time for both, is the exact same time,
since by the time they've covered 65 miles, let's say is
11:30am, is the same 11:30am for the first biker as well
as the second biker



now... do you see the substitution there?
so.. do that, and solve for "t"