What would the selling price of each item be

Number 6 is wrong. It is a decrease of 13/19 or roughly 68.42% decrease.

Number 7)
Cost of computer: $899.95
If there is a discount of 40%, that means you are only paying 60% of original price.  So we multiply by 0.6.

And if there is a tax of 4%, that means we will have to pay 104% of the final cost.  So we multiply by 1.04.

Thus the answer is:

899.95 * 0.6 * 1.04 = $561.5688 or $561.57

If your asking how much altogether for 12 performance the answer will be 98,496.