I=PRT
Interest=I
P=principal
R=rate in decimal
T=time in years
given
asuming 640 is the principal and not the interest
P=640
R=3%=0.03
T=2
I=640*0.03*2
I=19.2*2
I=38.4
interst=$38.40
If you sleep at 8:25pm and wake up at 6:15am you slept for 9hrs and 45 minutes.