Osmotic pressure needed for water absorption in blood capillaries

[perulv]

Homework Statement

The osmotic pressure (π) of the blood plasma necessary to induce absorption of water from interstitium (outside) to the capillary bed under circumstances shown on the picture (attached) has to be
a. Less than 20 mmHg
b. Between -1 and 20 mmHg
c. Equal to 21 mmHg
d. Greater than 21 mmHg

Homework Equations

see below

The Attempt at a Solution

I'm guessing the correct answer is (c) equal to 21.
Something like (20+1)-(?-0)... Am I on the right track, or completely wrong?

 

[perulv]

No one knows?