In the morning, John drove to his mother's house in the

statement 1. as distance is constant then we can apply direct formula 2xy/x+y= average rate here x=60 and y=40 then putting the values in formula we get 2*60*40/60+40= 48km/hr. hence suff.

statement2. no info. about another rate form village to town so insuff.

We are given that John drives at a rate of 60 km per hour and drives at a lesser rate when driving again later. We also are given that the distances driven are the same and need to determine the average rate.

We can use the formula:

average speed = total distance/total time

Statement One Alone:

In the evening, John drove at a constant speed of 40 kilometers per hour.

Since the distance each way is d, we can let time 1 = d/60, and time 2 = d/40; thus:

average speed = 2d/(d/60 + d/40)

average speed = 2d/(2d/120 + 3d/120)

average speed = 2d/(5d/120)

average speed = 240d/5d = 48

Statement one alone is sufficient to answer the question.

Statement Two Alone:

John's morning drive lasted 2 hours.

Knowing only the total time is not enough to determine the average speed.

Answer: A

ScottTargetTestPrep

Hi, can we use the formula 2S1S2/(S1+S2) to solve st. 1?

As a matter of fact, you can. In the formula average speed = 2d/(d/60 + d/40) that I used above, we can factor out d in d/60 + d/40 and we will get:

2d/[d*(1/60 + 1/40)]

If we cancel d from the numerator and the denominator, we get:

2/(1/60 + 1/40)

Adding the two fractions in the denominator, we have:

2/((40 + 60)/(40*60)) = (2*40*60)/(40 + 60)

This is the exact same expression that you would get if you took S1 = 60 and S2 = 40 in your formula. You should note that this is only applicable when the distances traveled are the same for both speeds.

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