- 30 Jul '08 09:51so im doing a few maths questions in preparation for this maths comp we have tommorow, and one question that was seemingly obvious had me stumped with the answer.

more or less, its ends up being a question of 7.6 - 16.7

easy yeah?

if u go thru the whole ordeal of setting up the subtraction table the answer wuld be -10.9 rite?

7.6

- 16.7

= -10.9

the actual answer: -9.1

WTH?

is there some rule that you cant subtract like this or something cuz if there is, i neva learnt it - 30 Jul '08 10:27

If you want to use the subtraction table, then use the following property:*Originally posted by banx99***so im doing a few maths questions in preparation for this maths comp we have tommorow, and one question that was seemingly obvious had me stumped with the answer.**

more or less, its ends up being a question of 7.6 - 16.7

easy yeah?

if u go thru the whole ordeal of setting up the subtraction table the answer wuld be -10.9 rite?

7.6

- 16.7

= ...[text shortened]... there some rule that you cant subtract like this or something cuz if there is, i neva learnt it

7.6 - 16.7 = -(16.7-7.6)

So just calculate 16.7 - 7.6 and add a minus sign in the end. - 04 Aug '08 08:06

A good example of why learning algorithms is bad practice unless you understand the mechanism behind it!*Originally posted by banx99***so im doing a few maths questions in preparation for this maths comp we have tommorow, and one question that was seemingly obvious had me stumped with the answer.**

more or less, its ends up being a question of 7.6 - 16.7

easy yeah?

if u go thru the whole ordeal of setting up the subtraction table the answer wuld be -10.9 rite?

7.6

- 16.7

= ...[text shortened]... there some rule that you cant subtract like this or something cuz if there is, i neva learnt it

With the subtraction algorithm you subtract .7 from .6 by 'borrowing' 1.0. That gives 1.6 - 0.7 = 0.9. But what does that 0.9 mean? It is POSITIVE 0.9.

Now the rest of the subtraction is 6 - 19 which gives NEGATIVE 10.

So the answer is the sum of NEGATIVE 10 and POSITIVE 0.9 which is as you found out is -9.1

Of course nobody would do it this way but it does work!