Don’t know if there’s much of an equation for it, but I guess the best way to write it out is 10 + x = -18. Then, you would subtract the 10 from both sides in order to isolate the x. So then you’re left with x = -28. So -28 is your answer.
You convert a decimal to a fraction by putting the digits to the right of the decimal point over "1" followed by the same number of zeros. (Most folks learn this in 3rd or 4th grade.)
0.66 = 66/100 . . . . . 2 decimals, so the denominator is 100
Then you reduce the fraction by canceling common factors. It helps to know your times tables and divisibility rules. (Or you can have your calculator do it.)
0.66 = 66/100 = 33/50
1.00 = 100/100 = 1
In order to solve this problem, we need to set up a proportion. I am setting the amount of calories in 10 ounces of grape juice equal to x.

We begin to solve this by cross multiplying.
8x = 1,100
To finish the problem, we divide both sides by 8.
x = 137.5, so the 10 ounce glass of grape juice will have 137.5 calories.
-8
f(x) is y, in which x is the independent variable
when x = 5, y = -8
Cost of Alejandro's food = $12
Tip paid by Alejandro = 20% of his bill = 20% of 12 = 2.4
Cost of Mia's food = $18
Tip paid by Mia = 15% of his bill = 15% of 18 = 2.7
Total Tip Paid = 2.4+2.7 = 5.1
Total Check Amount = 12 + 18 = 30
Total tip paid by Alejandro and Mia is r% of the total check amount.
So, we need to determine what % of 30 is equal to 5.1%
⇒ r% × 30 = 5.1
⇒ r% = 5.1 / 30
⇒ r% = 17%
Hence, r is 17%