Problem Solving

A certain car increased it's average speed by 5 miles per hour in each successive 5 minute interval after the first interval, if in the first 5 min interval it's average speed was 20m/h, how many miles did the car travel in the 3rd 5 min interval ans=2.5

I bet this one is more intimidating in the wording than in practice, but that's the GMAT for you.

In every 5 minute interval, the car increases speed by 5 mph. The first interval, the car is traveling 20 mph and we want to know the DISTANCE the car travels IN the third 5 minute interval.

To figure distance, we need to know RATE and TIME. The time intervals are constant at 5 minutes so we need to figure the RATE in interval 3 to calculate distance in interval 3.

Interval 1: RATE = 20 mph (given)
Interval 2: RATE = 25 mph (problem says it'll increase 5 mph in each successive interval)
Interval 3: RATE = 30 mph

So, to calculate distance (D=RT), D = 30 mph * 5 minutes. Since the units of time aren't the same, you have to fudge with one so you can do either:

(1) D = 30 mph * 5/60 hour (or 1/12 hour)
(2) D = 30/60 miles per minute * 5 minutes

Either way, you get 2.5 miles

Hope this helps, good luck!

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