Answer:
It will take 21 years.
Explanation:
First, find what x would be for it to equal 40 feet. This can be done with the equation 4.5 + 1.75x = 40.
4.5 + 1.75x = 40
Subtract 4.5 from both sides. This cancels out the +4.5.
1.75x = 40 - 4.5
Subtract 4.5 from 40 to get 35.5.
1.75x = 35.5
Divide both sides by 1.75. This cancels out the *1.75.
x = 35.5/1.75
Expand 35.5/1.75 by multiply both the numerator and denominator by 100.
x = 3550/175
Reduce 3550/175 by canceling out 25.
x = 142/7 = 20 2/7
Go up by one from 20 to get the answer of 21 years.