Answer :
s.p =rs.72
loss%= 10%
cp in terms of 100=100
sp=90
in real situation cp= x
sp=72
cross multiply and u will get the answer
let CP = x
loss% = loss * 100/ CP
10 = loss*100/x
x = 10*loss
loss = x/10
SP = CP - loss
72 = x - x/10
72 = (10x-x)/10
72*10 = 9x
x = 80
hence CP = Rs.80
loss% = loss * 100/ CP
10 = loss*100/x
x = 10*loss
loss = x/10
SP = CP - loss
72 = x - x/10
72 = (10x-x)/10
72*10 = 9x
x = 80
hence CP = Rs.80