$(20$ pts$)$ Calculate the time$-0$ present value of the following two streams of cash flows

$($CFs$).$ Assume $5\%$ interest rate per period. $CF(t)$ means the cash flow at period $t.$

a$.$ pts$)CF(0)=$$$10,CF(1)=$$$55,CF(2)=$$$70.$ Starting from period $3($including

period $3),CF$ is $$100$ forever.

b$.(10$ pts$)CF(0)=$$$5.$ Starting from period $1($including period $1),CF$ is $$10$ forever,

except for period $15,$ when the $CF(15)=$$$20.$