He needs 10 ml of the 30% solution, and 90 ml of the 10% solution. This will create 100 ml of 12% solution.
This is basically a system of equations problem. You can set it up like this:
Let x represent the amount of 30% solution needed and let y represent the amount of 10% solution needed
We know that 30% of x added to 10% of y must give us 12 ml of the acid. We also know that x and y must add to 100. Those are represented above.
Now you can use substitution to solve. In the second equation, x+y=100, we can solve for x to get x = 100 - y. Then if we sub that in to the first equation, we can solve:
We need 90 ml of the 10% solution, meaning we must need 10 ml of the 30% solution.