The answer to 12x23 is 276
Mitch did not line the place values up right
Let's say that the value invested in the account with a rate of 8% is "x", then the amount invested in the account with a rate of 12% is:

To calculate the total interest we need to calculate the interest of each individual account and sum them:

The total interest is the sum of the two expressions above:

The value invested in the account with 8% interest is 830, the one invested in the account with 12% interest is 1280.
The answer to number 18 is - 0.25 or 1/4
If you need help with the other ones ask.
<em>Hope this will help :)</em>