100 / 15

[PJMaybe]

Can someone tell me why I'm wrong here please.

100 / 15 = 6.7

But when I worked it out in my head I did it like this:
100/10 = 10
100/20 = 5
therefore 100/15 = 7.5

why is it wrong when I do it like that?

 

[stapel]

Can someone tell me why I'm wrong here please.

100 / 15 = 6.7

But when I worked it out in my head I did it like this:
100/10 = 10
100/20 = 5
therefore 100/15 = 7.5

why is it wrong when I do it like that?

You're assuming that the relationship is linear, and it isn't. It would be like assuming that 1/3 is exactly midway between 1/2 and 1/4, but of course 4/12 is not halfway between 6/12 and 3/12.

 

[e=mc²]

You're basically saying that 100 / 15 = (100 / 10 + 100 / 20) / 2, when in reality 100 / 15 = 100 / [(10 + 20) / 2], which gives you 100 / 15 again, while the first is saying that 100 / 15 = 15 / 2, which is wrong since they aren't equivalent.