Let's first figure out how much the tax is in dollars.
We see that Beth will be tipping a value of 15% of her $12 meal. Remember that in math, "of" almost always means to multiply, so we're multiplying 15% by 12:
15% * 12 = 15/100 * 12 = 0.15 * 12 = $1.80
So, the tip is $1.80. Add this to the original $12 to get the total cost:
20 is the greatest common denominator. You can find this by listing the factors of each number. The highest number that can fit into both individual numbers is the common denominator. Since 40 is just 20 doubled, and 20 is just 20 once, it fits into both numbers and is also the highest number that can fit.
