Question 12351
The 1 comes from 100%.  In your first problem, you said that the TOTAL bill, including the tip was $41.40.  This means that you have the cost of the meal, which is 100% of the meal PLUS 15% of the meal for the TIP.  That would be 115%.


Another way of looking at this is to say

Let x = cost of the meal.
.15x = amount of the tip (since it is 15% of the meal cost)


The equation is based upon the fact that the total is
 x + .15x = 41.40


Do you see that the x in this equation represents 1x?  That's where the 1 comes from!!


In the case of a DISCOUNT, you would be subtracting the discount from 100% instead of adding it.  This is apparently how you came up with the 98% in your second illustration.  However, take another look at that.  I think your second illustration should ALSO be an add-on, since it began with x at the beginning of the month, and then it grew 2% to end up with $870.  I think that should be the same situtation as when you added the tip to end up with a total amount.  


R^2 at SCC