Explanation:

720

there is simple interest thus the interest of every year will be equal

1020 becomes 1200 in 5th yr to 8th yr. it means in 3 year

so the interest in three year is= 1200-1020=180

thus interest of 5 yr = 300

5 year ago principle= A- I

                                   =1020-300

                                    =720