two F.1 questions of maths

If the selling price of the watch is $230,the profit% is 15%

What would be the profit% if the watch is $250?

Let the cost be C
(230-C)/C=15%
230C-C=0.15C
1.15C=230
C=$200
Profit = (250-200)/200*100%=25%
---------------------------------------------------------------------------
If a machine is $7760,the loss% will be 3%

a)find out the loss% if the machine is $7440
Let the cost be C
(7760-C)/C=-3%
7760-C=-0.03C
0.97C=7760
C=$8000
Loss=(7440-8000)/8000*100%=7%

b)what is the selling price if it is sold at a profit of 10%?
selling price=8000*(1+10%)=8000*1.1=$8800

1) The profit of the watch % : ((250-230(1-15%)) / 250 )*100%= 21.8%
----------------------------------------------------------
a) (1-7440(1-3%) / 7760) * 100% = 7%
b) 7760 (1+10%)/(1-3%) = 8800

Original cost = $230 / 1.15 = $200
Profit % for selling at $250 = ($250-$200) / $200 = 0.25 = 25%

Original cost of the machine = $7760 / 0.97 = $8000
a) Loss % = ($8000 - $7440) / $8000 = 7%
b) Selling price at a profit of 10% = $8000 X 110% = $8000 X 1.1 = $8800

Profit= 21.8%
---------------------------------
a) 6.92%
b) $8792.08