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 ?

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
$$\eqalign{ & = \left( {\frac{{604}}{{6800}} \times 100} \right)\% \cr & = 8\frac{{15}}{{17}}\% \cr} $$