Part a.
In order to solve this, we can start by calculating the raise per student, since $900 are needed to raise and there are 10 students, we can calculate the raise per student by dividing $900 by 10, like this:
Then, the raise per student is $90. By dividing this by the number of days, we get the raise per student per day, like this:
raise per student per day = 90/15 = 6
Then, the raise per student per day is $6, since each week has 7 days, we can multiply this value by 7 so we can get the raise per student per week, like this:
raise per student per week = 6×7 = 42 and we get:
Part b.
Similarly, if $1680 are needed and there are 21 students, we get:
$raise per student = total raise/students = 1680/21 = 80
Then, the raise per student is $80 And the raise per student per week for 40 days is: