2000(1 – 0.75) = 2000 (.25)

= 500 survivors after the dose of antibiotic a is taken

Then,
500(1.40)3 = 1374 bacteria after the 3 hours period at the instant before the second dose is taken.

a.) For antibiotic A, determine the bacterial population after 6 hours (before the third dose is taken)

That would be (1374*.25)(1.40)^3 = 943