Question

1. in 10 years, a child will be 3 years older than twice her current age. what is the current age of the child?
2. in 2007, there were 100 unhealthy air quality days in los angeles. this is 1 more than 3 times the number of unhealthy air quality days in san diego. how many unhealthy air quality days were there in san diego?
3. in 1995, u.s. hydroelectric power production reached 311 billion kilowatthours. this is 189 billion kilowatt - hours less than twice the 2009 production level. find the amount of hydroelectric power produced in 2009.

use the formula d = rt to find the value of the missing variable.

1. r = 4mph, t = 2hrs, d = ___
2. d = 1000ft, t = 50sec, r = ___
3. d = 200mi, r = 40mph, t = ___
4. r = 70mph, t = 2.5hrs, d = ___
5. a person driving a car at a constant speed for 4 hours and 15 minutes, travelling 255 miles. find the speed of the car in miles per hour. (ex. 4 on notes)

Explanation:

9.

Step1: Let the current age be $x$.

In 10 years, the age will be $x + 10$. And it is given that $x+10=2x + 3$.

Step2: Solve the equation for $x$.

Subtract $x$ from both sides: $x+10 - x=2x + 3-x$, which gives $10=x + 3$. Then subtract 3 from both sides: $10-3=x+3 - 3$, so $x = 7$.

Step1: Let the number of unhealthy - air quality days in San Diego be $x$.

The number of unhealthy - air quality days in Los Angeles is 100, and $100=3x + 1$.

Step2: Solve the equation for $x$.

Subtract 1 from both sides: $100 - 1=3x+1 - 1$, so $99 = 3x$. Then divide both sides by 3: $\frac{99}{3}=\frac{3x}{3}$, and $x = 33$.

Step1: Let the 2009 production level be $x$ billion kilowatthours.

The 1995 production is 311 billion kilowatthours, and $311=2x-189$.

Step2: Solve the equation for $x$.

Add 189 to both sides: $311 + 189=2x-189 + 189$, so $500 = 2x$. Then divide both sides by 2: $\frac{500}{2}=\frac{2x}{2}$, and $x = 250$.

Answer:

7

10.