Explain how you can use estimation and mental math to calculate a 20% tip.

Say that you have a bill for x dollars.  then a 20% tip is .2x

.2x is the same as 2 * (.1x)

If you recall, however, dividing by 10 simply involves moving the decimal place one position to the left (if you owe $31.74 for a bill, 10% is 3.17, then multiply by 2 to get $6.34 as approximately 20%), then double it.

This will get you the correct approximation of the tip up to about a cent, and it tends to be relatively easy to divide by 10 (move a decimal) and to double something in your head.  Then you just add that onto the bill.

x = ±sqrt(10)

step-by-step explanation:

5 - 2x² = -15

subtract 5 from each side

5-5 - 2x² = -15-5

-2x^2   = -20

divide by -2 on each side

-2x^2   /-2= -20/-2

x^2 =10

take the square root on each side

sqrt(x^2 )=±sqrt(10)

x = ±sqrt(10)

check:

5 - 2x² = -15

+sqrt(10)

5 - 2(sqrt(10))² = -15

5 -2(10) = -15

5 -20 = -15

-15 = -15

-sqrt(10)

5 - 2(-sqrt(10))² = -15

5 -2(10) = -15

5 -20 = -15

-15 = -15

to find the total number of combinations, multiply the quantity of each item by each other:

2 pairs of shoes x 3 pairs of shorts = 6 different combinations.