Hey! To solve this task, you need to know what value each variable has. Let's see:

p - principal =1000

a - bank account : If you earned 120 in 1 year you must have 1120

t - time in years you need to get a minium interest rate ..t=1

n - number of compounding periods in a year. n=4

Now we have an equation:

a=p(1+r/n)^nt you get 1120=1000(1+r/4)^4*1

the equation for r:

1120/1000=(1+r/4)^4

1.12^1/4=(1+r/4)

4(1.12^1/4-1)=r