You travel 12 miles. How much fuel will you need if you burn 150 kgs per hour, and are travelling at 90 mph?
I have a big doubt: my reasoning is the following:
Travelling at 60mph--> 12min for 12 miles
Travelling at 120mph--> 6 min for 12 miles
Yet, if I travelled at 90mph--> 9 min for 12 miles, shouldnt that be right?????, proportional calculations.
Well no, the solution is 20, which is 0,1333 (8minutes) X 150 and not 0,15 (9minutes).
Why is this???
when do I know if I can apply or not the correlative calculation?
You have to look at the proportions slightly differently.
If you're going from 60 mph to 120 mph you are multiplying by 2. Therefore to calculate the change in minutes you multiply by (1/2) [2 x 1/2 = 1]. So 12 minutes x 1/2 = 6 minutes. This, as we know, is correct.
Therefore:
If you're going from 60 mph to 90 mph you are multiplying by 3/2. Therefore to calculate the change in minutes you multiply by (2/3) [3/2 x 2/3 =1]. So 12 minutes x 2/3 = 8 minutes.
I hope this makes sense.