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Start with the given function.

Plug in

.

Multiply -0.6 and 15 to get -9

Raise "e" to the negative 9th power to get 0.000123

Multiply 19.9 and 0.000123 to get 0.00245

Add 1 to 0.00245 to get 1.00245

Divide
So rounding to the nearest whole number (since 0.445 of a person doesn't make sense), we get 3,491 people.
So after 15 days, about 3,491 people are infected.