A man walks from place A to place B at the speed of 3 km/h and cycles immediately to place A along the same road at a speed of 15km/h. Find the average speed for the whole trip.
If x^2+y^2=5 and x+y=3, find the value of x-y.
If 3a=2b=5c, find 1/a:1/b:1/c.
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