This is how I'd set it up:
405/20 = 243/x 
this means the mpg (miles / gallons) for 1 situation is the same as the mpg for the other situation (because the rate is the same)
then just cross multiply and solve:
405x=243*20 or 405x=4860 or x = 12
he would need 12 gallons of gas for 243 miles
        
             
        
        
        
G[f(-1)]
-2x[3-x(-1)]
-2x[-3-x]
6x-x
5x
        
             
        
        
        
Answer:
  Sam made it -24 versus 24  (29-5=24  not -24)
(Sam failed to flip the inequality)
Step-by-step explanation:
-4x + 5 > 29
Subtract 5 from each side
-4x+5-5 > 29-5
-4x > 24      Sam made it -24 versus 24  (29-5=24  not -24)
Divide by -4  Remember to flip the inequality
-4x/-4 < 24/-4   (Sam failed to flip the inequality)
x < -6
 
        
                    
             
        
        
        
 Circumference = 2×3.14×r
3.14 is pie and r is the radius.
To find the radius you divide the diameter by 2.
19 ÷ 2 = 9.5
2×3.14×9.5 = 59.66 
Multiply 59.66 by 6 because ther are 6 pennies.
59.66 × 6 = 357.96 mm.