Annual oil production for a region was about 540 million tons in 1950 . Production increased at a rate of $7.3 \%$ per year between 1950 and 1973 , but the rate of growth then slowed. The oil production reached approximately 4 billion tons per year in 2020 . Complete parts (a) through (d) below.
a. Using the growth rate of $7.3 \%$ for the period 1950 to 1973 , approximately what was the oil production in 1973 ?

The oil production was approximately $\square$ billion tons in 1973.
(Round to one decimal place as needed.)

Step 1 ：Given that the initial amount of oil production (P) is 540 million tons, the growth rate (r) is 7.3% or 0.073 in decimal form, and the growth is compounded once per year (n=1). We are looking for the oil production in 1973, which is 23 years after 1950 (t=23).

Step 2 ：We can use the formula for compound interest, which is also applicable to growth rates. The formula is: \(A = P(1 + r/n)^{nt}\)

Step 3 ：Substitute the given values into the formula: \(A = 540(1 + 0.073)^{23}\)

Step 4 ：Simplify the expression inside the parentheses: \(A = 540(1.073)^{23}\)

Step 5 ：Calculate the power: \(A = 540 * 8.6\)

Step 6 ：Multiply to find the final answer: \(A = 4644\) million tons

Step 7 ：So, the oil production was approximately \(\boxed{4.6}\) billion tons in 1973.