Use the rule of 70 to estimate the time it will take an initial quantity to drop to half its value when h (x) = 20*0.97^x, where x is months

The rule of 70 is used to estimate the time it will take an initial quantity to drop to half its value:

70 : (the decreasing rate) = x (months)

The decreasing rate = 1 - 0.97 = 0.03 = 3%

x = 70 : 3 = 23.33 = 23 1/3 = 23 months and 10 days.

Verify:

20 * 0.97^ (23.33) = 9.83 ≈ 10