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:
![\frac{\text{\$}900}{10\text{students}}=\text{\$90 per student}](https://img.qammunity.org/2023/formulas/mathematics/high-school/ngp8b7xqmnob4fawltm3hbkj3rcwfhcwe2.png)
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:
![\frac{\text{\$42 student}}{1\text{week}}](https://img.qammunity.org/2023/formulas/mathematics/high-school/3z314rasrcswu42gk5zj2j69c86o9mped9.png)
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:
![\frac{\text{\$80student}}{40days}*\frac{7\text{days}}{1\text{wek}}=\frac{\text{\$14tudent}}{1\text{week}}](https://img.qammunity.org/2023/formulas/mathematics/high-school/tl132jvuvsyi0cr0djld5hg87egdk4o4we.png)