A pension plan has an obligation to pay NOK $10,000$ once a year during a $10-$year period, beginning

$5$ years from now. The interest rates are flat at $10\%$

a$)$ what is the duration of this obligation?

b$)$ If the pension fund intends to use a $5-$yr and $20-$yr zero coupon bonds to immunise this position,

how much should be placed in each bond?