Gold in 20 times as heavy as water and copper is 10 times as heavy as water. In what ratio these two metals be mixed so that the alloy is 13 times as heavy as water is                                                                                    Options :A] 2:7B] 3:7C] 1:2D] 2:3

This problem can be solved using the concept of weighted averages.

Let's denote the weight of gold as G, the weight of copper as C, and the weight of the alloy as A.

We know that: G = 20 (since gold is 20 times as heavy as water) C = 10 (since copper is 10 times as heavy as water) A = 13 (since the alloy is 13 times as heavy as water)

We want to find the ratio of gold to copper (G:C) such that the weight of the alloy is 13 times as heavy as water.

We can set up the following equation using the concept of weighted averages:

(G * weight of gold + C * weight of copper) / (weight of gold + weight of copper) = A

Substituting the given values, we get:

(20 * weight of gold + 10 * weight of copper) / (weight of gold + weight of copper) = 13

Solving this equation for the ratio of gold to copper (weight of gold / weight of copper), we get:

(20 - 13) / (13 - 10) = 7 / 3

Therefore, the ratio of gold to copper should be 7:3, which is not an option in the given choices. There might be a mistake in the problem or the options provided.