While purchasing one item costing Rs. 400, I had to pay the sales tax at 7% and on another costing Rs. 6400, the sales tax was 9%. What percent of the sales tax I had to pay, taking the two items together on an average ?

Answer: Option C

Total sales tax paid = 7% of Rs. 400 + 9% of Rs. 6400
= Rs. $$\left(\frac{7}{100}\times 400+\frac{9}{100}\times 6400\right)$$
= Rs. (28 + 576)
= Rs. 604
Total cost o the items :
= Rs. (400 + 6400)
= Rs. 6800
∴ Required percentage
$$\begin{array}{rl}& =\left(\frac{604}{6800}\times 100\right)\%\\ & =8\frac{15}{17}\%\end{array}$$