Math CentralQuandaries & Queries


Question from Roy, a student:

May I ask if it is correct to add the following with units included? Some tell me that the units cannot be included in the addition because we cannot add speed.

50 km/h + 10 km/h = 60 km/h

The question: A car was travelling at an average speed of 50 km/h and it increased its speed by 10 km/h.

The question is not about adding the speeds of two different vehicles so I don't see what's wrong with using the units here.

Hi Roy,

If I am driving my car at 50 km/h and increase my speed by 10 km/h then my speed d now is 50 km/h + 10 km/h = 60 km/h. In this situation it is appropriate to add the speeds. But this is not the situation you are in. The car was tavelling at an AVERAGE speed of 50 km/h. I don't know what my speed is at this moment but I increase my speed by 10 km/h. Since I didn't know my speed when I increased it I don't know what my new speed is.

It's not the units that concern me in what you sent it's that you didn't ask a question so I am having trouble responding. Have a look at Penny's response to an earlier question. Does it help?

Write back if you need more assistance.


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