principal is p = 5000 $
rate of interest is 7 % per day
so the total interest will be
interest on 5000$ on 7 % interest for 20 days + 3000 $ on 7% for 40 days
now interest is
so she finally paid 15400 + 5000 =20400 $
if she does not make a partial payment then interest will be'
difference the interest
21000 - 15400 = 5600 $
so she saves 5600 $ by making the partial payment of 2000$ on 20th day.