She wrote a 4 instead of a 3.
There are only 3 numbers that are greater than 7. They are 8, 9, and 10. Mariah must have include 7 in her list. That is how she got 4 instead of 3.
The answer is 30.25 or as a fraction 30 1/4
Answer:
a) 
b) 
Step-by-step explanation:
By definition, we have that the change rate of salt in the tank is 
, where 
is the rate of salt entering and 
is the rate of salt going outside.
Then we have, 
, and
So we obtain. 
, then
, and using the integrating factor 
, therefore 
, we get 
, after integrating both sides 
, therefore 
, to find 
we know that the tank initially contains a salt concentration of 10 g/L, that means the initial conditions 
, so 
Finally we can write an expression for the amount of salt in the tank at any time t, it is
b) The tank will overflow due Rin>Rout, at a rate of 
, due we have 500 L to overflow 
, so we can evualuate the expression of a) 
, is the salt concentration when the tank overflows
