5 = 75 / C.P. x 100 will not work because 75 is not net profit (i.e. S.P. - C.P.)
If 75 is considered as a profit, then using formula S.P. = C.P. + (P%) x C.P, it will lead to C.P. = 1500.
Here, earning is difference between two transactions, with 10 % loss and 5 % profit.
Therefore, 75 = (S.P. with 5 % profit) - (S.P. with 10% loss)
75 = (C.P. + 0.05 x C.P.) - (C.P. - 0.1 x C.P.)
75 = (1.05 x C.P.) - (0.9 x C.P.)
75 = 0.15 x C.P.
C.P. = 500