A man has to travel 50km in 4 hours. He does it by walking the first 7km at x km/h, cycling the next 7km at 4x km/h and motoring the remainder at (6x + 3) km/h. Find x.
So we now know hat the
total distance and
average speed is 50 km and 4 hrs respectively, that is we can find the distance for the last leg (as the distances have to add to 50).
Now we also know that the total time is 4 hrs, so we need to rearrange the speed formula for time so that we can fidn the time taken for each leg, add it together and let it equal the total time.
Let t
1 be the time taken for leg 1, t
2 be the time taken for leg 2 and t
3 be the time taken for leg 3.
Now remember that the total time is 4 hours, so they must add to 4 hours.
Then substitute.
+36(4x)=4*4x(6x+3))
(16x+5)=0)
or
(Km) (as x>0)
Hope thats right.
Now there isn't really a set way to approach each of these questions as they do change. The best way to do it is identify what you are looking for, from that you will normally need to find the
total time,
total distance or
average speed (which is total distance/total time) in some cases like the above this will be given, but you will need to find the relevant parts that add to it, i.e. in the above the times taken for each of the three legs. This involves the rearranging of the speed formula. Once you get those parts and add them together you usually get fractions added together (not always though) and then cross multiply, combine them e.t.c. You may also need to run simultaneous equations with some questions, but again, it just involves working out what you are looking for, what you need to make that (manipulating the speed formula), then the parts/legs that go together.
Probably made that a big too long and partially repetitive, but there isn't a set method for all speed time distance questions, as they are not always missing the same bit of data.
Hope that helps
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