Answer:
He used 20 ounces of the 10% alloy.
Explanation:
Let x represent the number of ounces of alloy containing 10%
silver that was used.
Let y represent the number of ounces of alloy containing 40%
silver that was used.
The weight of the alloy made is 60 ounces. It means that
x + y = 60
The amount of silver contained in the first alloy is 10/100 × x = 0.1x
The amount of silver contained in the second alloy is 40/100 × x = 0.4x
The amount of silver contained in the mixture is 30/100 × 60 = 18
Therefore,
0.1x + 0.4y = 18- - - - - - - - - - - - -1
Substituting x = 60 - y into equation 1, it becomes
0.1(60 - y) + 0.4y = 18
6 - 0.1y + 0.4y = 18
- 0.1y + 0.4y = 18 - 6
0.3y = 12
y = 12/0.3
y = 40
x = 60 - 40
x = 20