An investor deposits $9000 into an account that earns simple interest at 5.45% annually. the amount a (in dollars) in the account is calculated using the formula nr019-1.jpg in the formula, p is the principal (in dollars), r is the interest rate (in decimal form), and t is the time (in years). how much interest is gained on the investment after 3 months?

We know that
the formula for calculating simple interest is
I=P*r*t
A=P*[1+r*t]
where 
I is the simple interest
A is the amount in dollars
P is is the principal (in dollars)
r is  the interest rate (in decimal form)
t  is the time (in years)

in this problem
P=$9000
r=5.45%> r=0.0545
t=3 months> t=3/12> t=0.25 year

substitute in the formula
A=P*[1+r*t]> 9000*[1+0.0545*0.25]> 9000*[1.013625]
A=$9122.625
I=P*r*t> 9000*0.0545*0.25> I=$122.625
or  also
interest =$9122.625-$9000> $122.625

the answer is
$122.625