Hey there!!
The question states that a sum is invested which is $3000 and hence, this would be counted as the principle amount or the principle sum.
The term given is 3 years, which gives us the idea that this interest is being counted or being active for 3 years.
She ( Diana ) earned the sum $450 at the end of three years. This means, the simple interest is $450.
Never get confused with the principle amount, principle sum and interest.
Principle amount = Principle sum + The interest.
Formula given :
I = prt
'p' is denoted as the principle sum, r is the rate and the t is the term or the number of years.
Now, plugging in all the values :
... 450 = 3000×r×3
... 450 = 9000×r
... r = 450 / 9000
... r = 0.05
... r = 5%
As we need the answer in the decimal form, the answer would be 0.05
Hope my answer helps!!