[15] A couple wants to accumulate $ 10000 and they deposit $ 500 at the end of each half year in an account paying i(2) = 5%. (a) [5] How many full deposits should they make? (b) [5] If the last regular deposit is increased by an amount (balloon deposit), what is the total amount of the last deposit? (c) [5] If a smaller concluding deposit is made one period after the last full deposit (drop deposit), how much is the concluding deposit? (Note that if the concluding deposit is negative after calculation, then the deposit is 0)