The given equation is

d is the distance in miles
t is the time in seconds
b. We need to find the distance in 30 seconds
Then substitute t by 30 in the equation

The zero in 30 we move the decimal point 1 place to right then

It will travel 4.5 miles in 30 seconds
c. We need to find the time to travel 12.75 miles
Then substitute d in the equation by 12.75

Divide 12.75 by 0.15 to find t

The decimal point up will cancel the decimal point down

It will take 85 seconds to travel 12.75 miles