Rounding in Exponential Qs (1 Viewer)

[kpq_sniper017]

Sometimes it's awkward to round to the "nearest" in exponential growth and decay questions. For instance:

The number of rabbits in a colony is given by N=80e^0.02t where t is in days.
b) How many rabbits will there be after 30 days.

My answer: 145.77 = 145
Answer in book: 146

Technically, my answer is right, because the population hasn't quite reached 146. However, rounding to the nearest whole rabbit takes it to 146.

Is rounding really that important in questions such as these? Will markers accept both answers? If not, which is the best "method of rounding" to use?

 

[acmilan]

id probably do it the way u did it

 

[redslert]

it's different from textbook to textbook

i remember doing one text book, maybe phonex by-topic MAYBE, even if the answer was 10.89 they would round it down to 10

but i remember doing question from tutor he rounded it up to 11
his argument was ... they could be deformed .... missing a arm, you know....but they are still a thing....yeh completely dodge but hmmm

the thing is, if you write down say 11.48 first and then round it either up or down you will still be correct because you did get the answer just not perfect...

i can't remember what i did now

i think i just rounded it down and then wrote next to the answer, "rounded down"

 

[CM_Tutor]

I think if you want to round 10.85 down to 10, you should explain what you did. ie.

calculation =
=
=
= 10.85 ... rabbits
approx = 10 rabbits, as you can't have a fraction of a rabbit, and so we must round down.

If in doubt about what a question means, explain. That is, write onto the paper a note saying "This can mean X or Y, and I am taking it to mean X", possibly with an explanation. I remember doing an exam with an ambiguous question - everyone who interpreted it differently from the way the examiner meant was penalised, except me. The reason was I wrote a note like this. The examiner wrote under it "this is an incorrect interpretation, but I accept your stated assumption". He wouldn't change anyone else's marks because he said he couldn't know if they had made a mistake. In other words, if you round 10.85 down to 10 without an explanation, how does the examiner / marker distinguish between the case where you think everything rounds down (wrong) and the case where you think you can't have 0.85 of a rabbit (reasonable).

 

[acmilan]

Good advice but have you any experience with HSC markers and know which way they go, up or down?

 

[CM_Tutor]

Don't know about the HSC specifically, but since most markers are teachers there is probably variability. Hence, cover yourself. You never know if you'll be marked by someone at 10:30 at night who's in a seriously bad mood and would like to tear some poor, hapless victim limb from limb by taking off marks.

 

[Xayma]

Hmm with decay would you normally round up or down (of course stating which way you went), eg if the population of rabbits is decreasing would you count 10.89 as 10 or 11, as you cant have .11 rabbits dead but its going the other way to the above (ie from 11)?

 

[kpq_sniper017]

so in order to take the easier and quicker way out, is it best to round to the nearest and put (nearest whole) next to it? it saves writing an explanation that may waste a minute or so....or if not....write both answers???

i think what i've done for some questions before when there was an ambiguity was to write two answers down. i did that with a question on a 2U test this year for simpson's rule (one using a table of values and one without).....lost a mark, protested, and got the mark

 

[CM_Tutor]

Originally posted by pcx_demolition017
so in order to take the easier and quicker way out, is it best to round to the nearest and put (nearest whole) next to it? it saves writing an explanation that may waste a minute or so....or if not....write both answers???

i think what i've done for some questions before when there was an ambiguity was to write two answers down. i did that with a question on a 2U test this year for simpson's rule (one using a table of values and one without).....lost a mark, protested, and got the mark

Writing two answers is a bad idea, esp. in the HSC - how does the marker choose which one to mark, and you can't argue about marks after the event.

There is bucketloads of free time in a 2u exam, so just take the time to explain what you did.

 

[skypryn]

This is something that everyone comes across and is a stupid argument. just put in brackets (nearest whole no.) and give the answer as exactly that. if you're rounding then the population doesn't have to actually have reached that number, beacuse you're just rounding off the answer, say for statistical purposes.

similarly, when doing any equation, rounding off doesn't give an exact answer - that's obviously not what the term "round-off" means. just always put (nearest rabbit) and you'll be correct 100% of the time