Average Speed = (Total distance traveled) / (Total time taken)
Let’s say a person travels different distances d1 with speed s1 in Time t1, d2 with speed s2 in time t2, d3 with speed s3 in time t3 … and dn with speed sn in time tn.
So, the average speed, using the above formula is:
(d1+d2+d3+d4+…+dn)/(t1+t2+t3+t4+…+tn) = (s1*t1+s2*t2+s3*t3+…+sn*tn)/(t1+t2+t3+t4+…+tn) = (d1+d2+d3+d4+…+dn)/(d1/s1 + d2/s2 + d3 /s3 + d4/s4 +…+ dn/sn)
Use any of the above 3 versions of the formula depending on the requirement.
Solve the following Data Sufficiency question-
How much time did it take a certain car to travel 400 kilometers?
(1) The car traveled the first 200 kilometers in 2.5 hours
(2) If the car’s average speed had been 20 kilometers per hour greater than it was, it would have traveled the 400 kilometers in 1 hours less time than it did