Cassidy has saved $8,000 this year in an account that earns 9% interest annually. based on the rule of 72, it will take about years for her savings to double.

The number of years in which saving gets double is 8 years .

Step-by-step explanation:

Given as :

The principal amount saved into the account = p = $8,000

The rate of interest applied = r = 9%

The Amount gets double in n years = $A  

Or, $A = 2 × p = $8,000 × 2 = $16,000

Let the number of years in which saving gets double = n years

Now, From Compound Interest method

Amount = Principal ×

Or, 2 × p = p ×

Or, $16,000 = $8,000 ×

Or, =

Or, 2 =

Now, Taking Log both side

2 =

Or, 0.3010 = n × 1.09

Or, 0.3010 = n × 0.0374

∴ n =

I.e n = 8.04 ≈ 8

So, The number of years = n = 8

Hence, The number of years in which saving gets double is 8 years . Answer

it has the same slope and a different y-intercept

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