Very simple maths question

Queen-Mise

I was working out some figures the other day, and just wanted someone to externally verify it.

If you are working out the % of something - in my case €912, I was taking 10% off and then 15% off. And then I did it the other way around. I got the same figure.
Is that correct that it doesn't matter which one you take off first as you get the same answer.

professore

Queen-Mise wrote: »

I was working out some figures the other day, and just wanted someone to externally verify it.

If you are working out the % of something - in my case €912, I was taking 10% off and then 15% off. And then I did it the other way around. I got the same figure.
Is that correct that it doesn't matter which one you take off first as you get the same answer.

You are taking in total 25% off, so if you do 10% and 15% or 15% and 10%, or even 10%,10% and 5%, you should always get the same answer.

Thomas_S_Hunterson

Queen-Mise wrote: »

I was working out some figures the other day, and just wanted someone to externally verify it.

If you are working out the % of something - in my case €912, I was taking 10% off and then 15% off. And then I did it the other way around. I got the same figure.
Is that correct that it doesn't matter which one you take off first as you get the same answer.

What does matter is if you first take off 15% of 912 and then take off 10% of the remainder or vice versa.

Pines

Sean_K wrote: »

What does matter is if you first take off 15% of 912 and then take off 10% of the remainder or vice versa.

Not correct, unless I misunderstand you...

Taking 10% off any given number is the same as multiplying by 0.9. [i.e. x - 0.1x = 0.9x]

Similarly, taking 15% off any given number is the same as multiplying by 0.85.

So if we start with 912, the answers are the same whichever operation we do first ((912 * 0.9) * 0.85) = ((912 * 0.85) * 0.9) = 697.68

Of course if the OP is looking to subtract 10% of 912, and 15% of 912 in succession ... then professore is right, and the answer is 684 whichever is done first.

Thomas_S_Hunterson

Pines wrote: »

Not correct, unless I misunderstand you...

Taking 10% off any given number is the same as multiplying by 0.9. [i.e. x - 0.1x = 0.9x]

Similarly, taking 15% off any given number is the same as multiplying by 0.85.

So if we start with 912, the answers are the same whichever operation we do first ((912 * 0.9) * 0.85) = ((912 * 0.85) * 0.9) = 697.68

Of course if the OP is looking to subtract 10% of 912, and 15% of 912 in succession ... then professore is right, and the answer is 684 whichever is done first.

Yes, sorry my post wasn't very clear, I was just hinting to the OP that they'd get a different result if they did want to compound the reduction.

Michael Collins

Trying to see if it's better to get the sales discount then the employee discount or vice versa is it

And who said maths wasn't useful...

red_fox

A, Pines is right, for the very reason said. (commutativity of multiplication)

B, If you take 15% of 912 and then 10% of the 912 you would get the same as 10% of 912 and then 15% of 912 (commutativity of addition)

But A does not equal B (in general).

A related question I was asked once, when is x% off the same as y% extra free? (I was actually asked if 25% off was the same as 25% extra free

it's not.

)

Queen-Mise

Michael Collins wrote: »

Trying to see if it's better to get the sales discount then the employee discount or vice versa is it

And who said maths wasn't useful...

I was buying a bed in Homebase and I had 10% of because of their card, and then 15% reduction when buying on Friday or Saturday.
Was wondering if there was any difference in what order the reductions were given. Was sitting here working it out... Hence thread.

thanks for the replies people, very appreciated.