Sue drove her car 428 miles in january 412 miles in february, and 270 miles in march. if her average milage for the four months from january to april is to be at least 328 miles, how many miles must she drive in april?

Sue drove 202 miles in April.

Explanation: To find the average, you would add up all the numbers and divide by the amount of numbers (4). Since there's a missing number that you need to find, you will basically do it backwards.

438 + 412 + 270 + X / 4 = 328
Multiply by 4 on both sides 
438 + 412 + 270 + X = 1,312
Add up the miles from January, February, and March
1,120 + X = 1,312
Subtract 1,120 on both sides
X = 202 

a) The initial number of infections refers to the time t = 0, not 1. At the start, there are

infected students.

b) Here you look for the time t such that P(t) = 600. We have

Rounding up, the school will close after around 19 days.