(d) 20%
Let C.P. = Rs c
\(\therefore\) C.P. of \(\frac{1}{4}th\) of the goods = \(\frac{\frac{c}{4}\times90}{100}\) = Rs \(\frac{9c}{40}\)
C.P of \(\frac{3}{4}th\) of the goods = Rs \(\frac{3c}{4}\)
Let profit on this remaining part = P%. Then,
S.P. of \(\frac{3}{4}th\) of the goods = \(\frac{\frac{3c}{4}\times(100+P)}{100}\)
= \(\frac{3c}{400}\)(100+P)
Profit on the whole transaction = 12.5%
\(\therefore\) S.P. of the whole = Rs \(\frac{c\times112.5}{100}\)
\(\therefore\) \(\frac{9c}{40}\) + \(\frac{3c}{4}\) + \(\frac{3c\times P}{400}\) = \(\frac{112.5c}{100}\)
\(\Rightarrow\) \(\frac{90+300+3p}{400}\) = \(\frac{112.5}{100}\)
\(\Rightarrow\) \(\frac{390+3p}{4}\) = 112.5 \(\Rightarrow\) 390 + 3P = 450
\(\Rightarrow\) 3P = 60 \(\Rightarrow\) P = 20.