Calculations Required: "In a traffic census at an accident black spot, 938 cars are seen to pass in one hour...."

[TheHoundsStranger]

"In a traffic census at an accident black spot, 938 cars are seen to pass in one hour. Of these, 300 are going at over 50 kilometres per hour (kph) and 638 are travelling more slowly. The average speed of these cars is calculated to be 36.4 kph, 51 kph or 16 kph?"

The answer is 36.4 but I'm unable to calculate how this figure is obtained. If 36.4 is multiplied by 938 this gives 34,143.20, but how do I get the figure of 34,143.20?

 

[Deleted member 4993]

"In a traffic census at an accident black spot, 938 cars are seen to pass in one hour. Of these, 300 are going at over 50 kilometres per hour (kph) and 638 are travelling more slowly. The average speed of these cars is calculated to be 36.4 kph, 51 kph or 16 kph?"

The answer is 36.4 but I'm unable to calculate how this figure is obtained. If 36.4 is multiplied by 938 this gives 34,143.20, but how do I get the figure of 34,143.20?

Is this the EXACT problem description that was given to you?

 

[TheHoundsStranger]

Hi. Yes, it's the text verbatim. Difficult to determine whether they want the average for the group in total (i.e. 938) or one of the groups individually. Either way, I'm can't figure it out.

 

[Dr.Peterson]

"In a traffic census at an accident black spot, 938 cars are seen to pass in one hour. Of these, 300 are going at over 50 kilometres per hour (kph) and 638 are travelling more slowly. The average speed of these cars is calculated to be 36.4 kph, 51 kph or 16 kph?"

The answer is 36.4 but I'm unable to calculate how this figure is obtained. If 36.4 is multiplied by 938 this gives 34,143.20, but how do I get the figure of 34,143.20?

Hi. Yes, it's the text verbatim. Difficult to determine whether they want the average for the group in total (i.e. 938) or one of the groups individually. Either way, I'm can't figure it out.

It's impossible to answer the question however you interpret it, since we don't know any actual speeds! But I'd expect it to be asking about all the cars.

The best I can imagine doing here would be to eliminate possibilities, assuming the question is meant to be about which of the three is a possible value for the average calculated with actual numbers.

For the slowest possible average, we can suppose that 300 cars go at barely over 50 kph and 638 go practically 0 kph; then the average would be (300*50 + 638*0)/938 = 15.99; so the average can't be less than that -- but 16 is conceivable (though highly unlikely).

For the highest average, we can suppose that 300 go as fast as you can imagine a car going (I'll suppose that is 150 kph), and 638 go just under 50. Then the average of all would be (300*150 + 638*50)/938 = 81.98.

So all three numbers are conceivable.

If all we do is guess at reasonable numbers, we might say that those going over 50 went 60, and those going under 50 went 40. Then the average is (300*60 + 638*40)/938 = 46.4. I'd say there are still two answers that are reasonable.

What is the context of this question? What have you been studying that it might be intended to test? What are the other questions about?