Rick has d dollars. the formula for the number of gallons of gasoline that rick can buy with d dollars is given by g = d/3. the formula for the numbers of miles that rick can drive on g gallons of gasoline is given by m =21g. how many miles can rick drive on $8 worth of gasoline?

G, the number of gallons, is a function of d, the dollars. The rule is :

g(d)=d/3.  For example, with 30 dollars, Rick can buy g(30)=30/3=10 gallons.

the number of miles that Rick can travel, m, is a function of g, described as follows:

m(g)=21g

for example, with 30 gallons, Rick can travel m(30)=21*30=630 miles.

now, with 8 dollars, Rick can buy g(8)=8/3 gallons of gasoline,

with 8/3 gallons, Rick can travel m(8/3)=21*(8/3)=7*8=56 miles

56 miles.