"The time horizon to be used when constructing a portfolio to pay for college expenses for a person who is expected to start college in 10 years and finish college in 15 years is:"

15 years

Explanation:

If you are constructing a portfolio to cover the education expenses of your child and you expect that he/she graduates from college in 15 years, then the time horizon of your portfolio should be 15 years since it should cover all the expenses until your child graduates. If you start a little earlier and expect your child to graduate in 20 years, the time horizon will be 20 years, or if you start a little later and expect your child to graduate in 10 year, then the time horizon is 10 years.

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