Given 3 Zero Coupon Bonds with Portfolio Weights 70,70 and -40 ( all units are Percentages ) with duration of 1,2,3 what is the total duration of the portfolio ?

Given 3 Zero Coupon Bonds with Portfolio Weights 70,70 and -40 ( all units are Percentages ) with duration of 1,2,3 what is the total duration of the portfolio ?