# Simple Math Problem

• Apr 26th 2012, 02:16 PM
phenomin
Simple Math Problem
Hi,

I have a what seems to be a simple math problem but for some reason the answer key is telling me my answer is incorrect. So here goes...

One motor scooter travels at an average speed of 48 kilometres per hour. A slower scooter travels at an average speed of 36 kilometres per hour. In 45 minutes, how many more kilometres does the faster scooter travel than the slower scooter?

My answer is 9km. I multiplied 48 by .75 which gave me 36. I then multiplied 36 by .75 which gave me 27. I subtracted the 27 from 36 to get the answer of 9. I figure 45 minutes is 3/4 or 75% aka .75 of an hour. Apparently, according to the answer key, the answer is 7.5km. Where did I go wrong?
• Apr 26th 2012, 06:00 PM
Soroban
Re: Simple Math Problem
Hello, phenomin!

Your work is excellent . . . They are wrong!

I noted that a small change in the problem would justify their answer.
If the faster scooter had a speed of 46 kph or the slower scooter had a speed of 38 kph,
. . the answer would be 7.5 kilometers.

Could you have typed the problem incorrectly?

• Apr 26th 2012, 06:13 PM
phenomin
Re: Simple Math Problem
Hi Soroban,

Thanks for your response. This was actually a question on a math paper my girlfriend was going over and when I gave her the answer she told me I was wrong. I checked the answer key and it did say 7.5km. I assure you I did not type it out incorrectly as it is typed identically word for word. At first I told her that her answer key is wrong and that sometimes these things happen but she seems to think they are never wrong, lol. Anyway, glad to have a second opinion from someone on a forum like this just to clarify. Thanks
• Apr 27th 2012, 09:45 PM
mathbyte
Re: Simple Math Problem
Hi phenomin-

You can also reason it this way: in one hour, the faster scooter covers (48-36) km, or 12 km, more than the slower one.
Thus, in 45 minutes, the difference in distance covered will be (45/60) x 12, or 0.75 x 12, which comes out to 9 km.
So, in case you wanted a third opinion... (Itwasntme)

An answer of 7.5 km is valid only if
1) The difference in scooter speeds in 10 km/h,
or
2) The time given is 37.5 minutes.