0.34 of 75 is 25.5, so we're looking for the probability that 26 or more of the 75 samples are credit sales (if I understand the question).

I would calculate this as

,

where n=75 and p=0.25.

The last time I did a calculation like this, I used Matlab. I saved the expression I used that time, so I could check your result just by substituting my numbers for yours. This is the result I got:

» p=0.25; k=26; n=75; s=0; for x=k:n; t=p.^x.*(1-p).^(n-x).*factorial(n)/(factorial(x)*factorial(n-x)); s=s+t; end; s

s =

0.0393

So I would say that your approximate answer is pretty close to the exact one.