[RESOLVED] MATHS PRO help pls

[SuperRookies]

A Maths Question :
example:
120 - 5% - 7% = 106.02

total discount = 11.5%

but my question is how to get the total discount?

[NickThissen]

120 - 5% isn't actually a proper equation. 5% of what? Of course you mean 5% of 120, but really it's ambigious.

120 - 5% should really be written as 120 - 0.05*120 = 120(1 - 0.05)
This amount is of course the amount you still have to pay, after discount.

The total discount then is of course just the original amount (120) minus the amount you still have to pay.
So, the discount is 120 - 120(1 - 0.05) = 120 - 120 + 0.05*120 = 0.05*120 = 5% of 120.

That was quite obvious from the 'equation' 120-5% of course. But now:

I am assuming that by 120-5%-7% you actually mean 5% discount of 120, and then 7% discount of that. You see now why your notation is ambigious? It could just as well mean 5% off 120, and then 7% off 120, which would just be 12% off 120.

So, 120 - 5% - 7% should be written as 120(1 - 0.05)(1 - 0.07) = 120(1 - 0.05 - 0.07 + 0.05*0.07) = 120(1 - 0.1165).
So, you still have to pay 120(1 - 0.1165). The discount is again the original amount minus that, so the discount is 120 - 120(1 - 0.1165) = 120 - 120 + 0.1165*120, or 11.65% of 120.

Can you follow?

[re_turner_jr]

to get the sale price after % discount

120 - (0.05 * 120) - (0.07 * (120 - (0.05 * 120)=
120 - (6)          - (0.07 * (120 - (6)         =
120 - (6)          - (0.07 * (114)              =
120 - (6)          - (7.98)                     =
        114        -  7.98                      =
        Ans = 106.02

to get total % off

((120 - 106.02) * 100) / 120 =
(     13.98 )   * 100) / 120 = 
                1398   / 120 =
        Ans = 11.65 %

If you prefer variables then

O = orig. price
A = first discount (in percentage)
B = second discount (in percentage)

O - (A * O) - (B * (O - (A * O))) = Y
((O - Y) * 100) / O   = Z

Y = the amount to be paid
Z = the total percent discount

[SuperRookies]

wow....
thanks a lot.... I fully understand now. very nice and short explaination