Question 643216
simple interest means no compounding.
this means your interest per month is going to be the same every month.
the formula for simple interest is:
f = p * i * n
i is the interest rate.
p * i is the interest
p is the principal.
f is the future value.
n is the number of time periods.
just solve for the monthly interest and work your way back.
in your problem:
account contains 780 after 8 months and contains 855 10 months after that.
in 10 months, the account grew from 780 to 855 which is a growth of 75 dollars.
divide that by 10 months and that becomes 7.5 dollars growth per month.
8 * 7.5 = 60 dollars.
subtract 60 from 780 and you start with 720 dollars 8 months before you had 780.
your original amount is 720 dollars.
your interest per month is 7.5
720 + 8*7.5 = 780 + 10*7.5 = 855.