The interest is $20 and the amount is $420.
Find an interest by using the formula I=P⋅i⋅t, where I is interest, P is total principal, i is rate of interest per year, and t is total time in years.
In this example P = $400, i = 5% and t = 1 years, so
I = P x i x t = 400 x 0 x 05⋅1 = 20
Now Find an amount by using the formula A=P+I.
Since P = $400 and I = $20 we have
A=P+I
=400+20
=420