Answer:
Z=X(v¹⁷+v¹⁵+v¹²) + Y(v²⁰+v¹⁸+v¹⁵)
Explanation:
For the the first child of age 1 yea to be paid an investment of X at age 18 will be Xv¹⁷ and an investment of Y at age 21 will be Yv²⁰·
also for then 3 years old child to be paid an investment of X at age 18 will be Xv¹⁵ and an investment of Y at age 21 will be Yv¹⁸
lastly for the 6 years old child be paid an investment of X at age 18 will be Xv¹² and an investment of Y at age 21 will be Yv¹⁵·
Hence the investment Z must be equal to the summation of all this values
Z=X(v¹⁷+v¹⁵+v¹²) + Y(v²⁰+v¹⁸+v¹⁵)