Answer: 3424.28
Step-by-step explanation:
Given data : 378, 361, 350, 375, 200, 391, 375, 368, 321
Number of data values = 9
Mean :

Variance = 

Variance = 
For this case, the first thing we are going to do is assume that all the tests are worth the same.
Then, we define a variable:
x: score of Mona's last test
We write now the inequality that models the problem:

From here, we clear the value of x:
Answer:
the lowest grade that Mona can get for her last test so that her test average is 90 or more is:
x = 87
I believe the answer is B because you multiply 18 and 40 then subtract 320 from that and you will 400.