bumblebee123
Junior Member
- Joined
- Jan 3, 2018
- Messages
- 200
Hello,
Please, could someone help me out with a question in my daughters' Math book. The question relates to direct proportion and is given in the form of information. It seems quite straightforward and I see how they get the answer but it's not how I get the answer and I don't know why they choose to put the letters where they do. The answer I get is wrong but why? I have copied it out exactly:
Dave is working out different ways of travelling around Europe. He is making maps showing the cities he might visit. The time, T minutes, that it takes Dave to draw a map is directly proportional to the square of the number of cities, c, he puts on a map. A map with 8 cities takes 10 minutes to draw.
Question a) Find a formula for T in terms of c.
How I would answer:
I start with:
T = kc^2
( I use ^2 to stand for the square, so its c squared)
I substitute the known numbers for cities and time taken to draw:
10 = k8^2
I continue to solve:
10 = k64
10/64 = k
Substitute the k in to the formulae to give the answer:
T = 10/64c^2
so, T is equal to 10 divided by 64 multiplied by c squared.
However, this is not the answer, instead the answer in :
T = c^2/6.4
So I figure they have done: c^2 = kT
in order to get their answer. But why have they done this when the question says to find a formula for T in terms of c? How would I know to put the constant of proportionality next to Time and not cities?
Your thoughts would be much appreciated.
Thank you in advance.
Please, could someone help me out with a question in my daughters' Math book. The question relates to direct proportion and is given in the form of information. It seems quite straightforward and I see how they get the answer but it's not how I get the answer and I don't know why they choose to put the letters where they do. The answer I get is wrong but why? I have copied it out exactly:
Dave is working out different ways of travelling around Europe. He is making maps showing the cities he might visit. The time, T minutes, that it takes Dave to draw a map is directly proportional to the square of the number of cities, c, he puts on a map. A map with 8 cities takes 10 minutes to draw.
Question a) Find a formula for T in terms of c.
How I would answer:
I start with:
T = kc^2
( I use ^2 to stand for the square, so its c squared)
I substitute the known numbers for cities and time taken to draw:
10 = k8^2
I continue to solve:
10 = k64
10/64 = k
Substitute the k in to the formulae to give the answer:
T = 10/64c^2
so, T is equal to 10 divided by 64 multiplied by c squared.
However, this is not the answer, instead the answer in :
T = c^2/6.4
So I figure they have done: c^2 = kT
in order to get their answer. But why have they done this when the question says to find a formula for T in terms of c? How would I know to put the constant of proportionality next to Time and not cities?
Your thoughts would be much appreciated.
Thank you in advance.