A lump of two metals weighing 18 g is worth Rs. 87 but if their weight is interchanged, it would be worth Rs. 78.60. If the price of one metal be Rs. 6.70 per gram, find the weight of the other metal in the mixture.

Answer: Option A

Cost of (18g of 1^st metal + 18g of 2^nd metal) = Rs. 165.60
Cost of (1g of 1^st metal + 1g metal of 2^nd metal) = Rs. 9.20
Hence cost of 1g of 2^nd metal,
= 9.20 - 6.70
= Rs. 2.5
Mean price = Rs. $$\frac{87}{18}$$

Now, $$\frac{\text{quantity}\text{of}{1}^{\text{st}}\text{metal}}{\text{quantity}\text{of}{2}^{\text{nd}}\text{metal}}$$     $$=$$ $$\frac{14}{6}:\frac{56}{30}=5:4$$
Quantity of 2^nd metal = $$\frac{18\times 4}{9}$$  = 8g