[Traumatic Acid]

Exam season is here! I've been working through a lot of practice questions in preparation. I'm very confused as to how my professor reached the answer to this question:

c. Given that the pH of your blood has not changed, the blood sample at that time was
actually a mixture of CO2 and HCO3-. From the information in a and b, calculate the
concentrations of CO2 and HCO3-.
Basically someone measured the pH of their blood to be 7.35, and after strenuous exercise the pH was still 7.35

The information from part A and B:
A: K3 of the reactions:  CO2 + H20 <--> H2CO3 <--> HCO3 + H⁺ is equal to 4.2x10^-7.
K1 = -1.7x10^-3
K2 = -2.5x10^-4
Which of course makes pK3 6.37

Part B:
From a blood sample of exactly 10 ml, 5.6 ml (2.5x10^-4 mols) of CO2 were extracted.

And this is where I get lost. I have attached a picture of the correct answer, but the working out is not complete. Where did the 9/1 come from? And how does that mean that the concentration of HCO3 is 0.024M? Likewise with CO2, (1/10)(0.025 M) =0.0025 ?? I don't understand where these numbers are coming from.
Also if anyone has any tips or methods that would help me understand and use the Henderson-Hasselbach equation, I'd love to hear it.
I have attached a picture of my previous attempts, which is pretty much just me flapping around not really knowing what to do, just so that you know I have tried.

Cheers in advance!

[Borek]

Where did the 9/1 come from?

This is all pretty simple math. Start by combining 7.35 and 6.4 on one side of the equation, then it is just applying properties of the log function.

How do you calculate x knowing that log₁₀(x)=2?