You want to put $8,000 in a simple interest account it has a 2% annual interest rate how long must you invest that money to earn $400 in interest

I=PRT
I=interst
P=present amount
R=interest rate
T=time in years

we know that
P=8000
R=2%=0.02

we also know that we want 400 in interst or I=400
we need to knw T

so
400=8000(0.02)(T)
400=160(t)
divide both sides by 160
2.5=T
2.5 years