Answer:
You must pick at least 13 pennies to be sure of getting at least five from the same year.
Step-by-step explanation:
You have:
12 1967 pennies
7 1868 pennies
11 1971 pennies.
how many must you pick to be sure of getting at least five pennies from the same year?
This value is the multiplication of the number of different pennies by the antecessor of the number of pennies you want, added by 1.
So
You have 3 differennt pennies
You want to get at least five from the same year.
You must pick at least 13 pennies to be sure of getting at least five from the same year.
For example, if you pick 12 pennies, you can have four from each year.
Adding three values, 13 is the smallest number that you need at least one term of the addition being equal or bigger than 5.