It is a first order isomerisation reaction. Integrated rate law for 1st order reaction is

$$kt = \ln {{{{[A]}_0}} \over {{{[A]}_t}}}$$ ...(i)

Here,

k = rate constant

$${[A]_0}$$ = initial concentration

$${[A]_t}$$ = concentration at time 't'

Given, k = 3.3 $$\times$$ 10^$$-$$4 s^$$-$$1

$${[A]_0} = 100 \Rightarrow {[A]_t} = 100 - 40 = 60$$

Put values in Eq. (i), we get

$$3.3 \times {10^{ - 4}}{s^{ - 1}} \times t = \ln {{100} \over {60}}$$

t = 1547.95 s = 25.79 min (1 min = 60 s or 1s = $${1 \over {60}}$$ min) = 26 minutes