Explanation:
x = number of km bus from A drives until they meet.
y = number of km bus from B drives until they meet.
x/80 is the time for the bus from A to reach the meeting point.
x/70 is the time for the bus from B to reach the meeting point.
both times must be equal, as they start at the same time.
so,
x/80 = y/70
but then we also know that together they have traveled the full 52 km to meet.
so,
x + y = 52
x = 52 - y
with that we go into the first equation :
(52 - y)/80 = y/70
(52 - y)/8 = y/7
7×(52 - y)/8 = y
7×(52 - y) = 8y
364 - 7y = 8y
364 = 15y
y = 364/15 km = 24.26666666... km
x = 52 - y = (780-364)/15 = 416/15 = 27.73333333... km
they will meet after (using bus from B)
364/15 / 70/1 = 364/1050 hours
= 182/525 hours =
= 60×182/525 = 10920/525 minutes =
= 728/35 = 104/5 = 20.8 minutes =
= 20 minutes 60×8/10 seconds =
= 20 minutes 48 seconds
control (using bus from A) :
416/15 / 80/1 = 416/1200 hours =
= 26/75 hours =
= 60×26/75 = 1560/75 minutes =
= 104/5 = 20.8 minutes
correct.