In general, the volume

has total derivative

If the cylinder's height is kept constant, then

and we have

which is to say,

and

are directly proportional by a factor equivalent to the lateral surface area of the cylinder (

).
Meanwhile, if the cylinder's radius is kept fixed, then

since

. In other words,

and

are directly proportional by a factor of the surface area of the cylinder's circular face (

).
Finally, the general case (

and

not constant), you can see from the total derivative that

is affected by both

and

in combination.
-41/25 I believe, I'm not sure why you put some addition signs in the inside of parentheses
If Gail practises swimming for 1 and 1/4 hours a day, then in a week she'll do:
1 1/4 * 7 = 5/4 * 7 = 35/4 = 8 3/4 [hours]
In other words she'll practice 8 hours and 45 minutes a week (assuming that she's still practicing on weekends).
M is a midpoint of BC so:

Length of MA:

Length of NB:
7(3r-1)-(r+5)=-52
21r-7-r-5=-52
21r-r-5-7=-52
20r-12=-52
Add 12 to both sides
20r=-40
Divide by 20
r=-2