Where did i go wrong?

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
= -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

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

If you want to use the subtraction table, then use the following property:

7.6 - 16.7 = -(16.7-7.6)

So just calculate 16.7 - 7.6 and add a minus sign in the end.

Haha! That's exactly what i told you banx99, and you weren't satisfied. Well, there you go! Oops too late the competitions over. XD

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

A good example of why learning algorithms is bad practice unless you understand the mechanism behind 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!

Originally posted by banx99
[b]so im doing a few maths questions in preparation for this maths comp we have tommorow.

Use a number line, the number line never lies.