William O. answered 04/10/19
Eccentric STEM/Testing Tutor With a Flair for Helpful Anecdotes
In this case, we need to understand what it means mathematically to receive a discount or to add a surcharge (like sales tax). When we add 5% of something, we are taking the original (say, a variable 'x'), and adding 5% of that vaiable's value to the total, so we would get something like this for the equation of just the sales tax:
y = x + 0.05x.
Now, if we use the distributive property of multiplication to factor the x out of the right side of the equation, we get y = x * (1 + 0.05) = x * 1.05. This means that a 5% sales tax is equivalent to multiplying the original number by 1.05.
Next, let's consider the discount. Similar to the tax, we are subtracting a percentage of the original variable from itself in order to get the answer, so we can put it into equation form like this:
y = x - 0.15x; but, by the same method by which we factored out the x in the sales tax problem, we can factor out the x in this problem as well:
y = x * (1 - 0.15) = x * 0.85.
So, we can see that, if we want to apply both a discount and a sales tax, we would just do the following:
y = x * 0.85 * 1.05
Now, you might be wondering "Does it matter whether I put the sales tax or the discount first?" Thankfully, it doesn't matter, because of the commutative property of multiplication. Thus, x * 0.85 * 1.05 = x * 1.05 * 0.85
Finally, we complete the multiplication, substituting 8.50 for x: x * 0.85 * 1.05 = 7.59
William O.
I think that Isidro below makes a good point, that it's grammatically ambiguous whether the ORIGINAL price of the meal is 8.50, or if it's the FINAL price of the meal. My method assumes that 8.50 is the ORIGINAL price of the meal, then you apply a 15% discount. Isidro's method assumes that the discount means "fifteen percent of original price", and that 8.50 is the FINAL cost of the meal. I disagree with his interpretation of the meaning of "discount", as his method would actually be an 85% discount. If you have any further questions about either of these answers, feel free to DM me.04/10/19