Explanation

The ratio of copper to silver is 3:7.

To find the percentage of silver in the alloy, we can use the following formula:

Percentage of silver = (Amount of silver / Total amount) × 100

First, let's find the total amount:

Total amount = Copper + Silver

= 3 + 7

= 10

Now, we can plug in the values:

Percentage of silver = (7 / 10) × 100 = 70%

So, the percentage quantity of silver in the alloy is 70%.

The correct answer is: 70%