There are two kinds of half-life questions.....
The first are ridiculously easy, like this one, where you have a small integer number of half-lives, in this case 4.
26 hr / 6.5 hr = number of half-lives
n --> n/2 --> n/4 --> n/8 --> n/16 ... amount of radionuclide left (n is the original amount)
.....1 ........2.........3...........4 ............ number of half-lives
Then, there are the realistic problems where there are more real-world situations. Radioactive decay is a first-order process and is described by this equation.
A = Ao e^(-kt) ...... where A and Ao are amounts, t is the elapsed time and k is the decay constant which is related to the half-life. k = ln2 / t½. (k = ln2 / 6.5hr) = 0.107 hr⁻¹)
A = Ao e^(-0.107 hr⁻¹ x 26 hr)
A = Ao (0.0625)
Since 0.0625 is the equivalent of 1/16, the answers to both methods are the same. Of course, if we knew what the original amount (Ao) of the isotope was we could get a more definitive answer.