Help understanding an old exam question

Hi!

Not sure where else to ask but I figured you guys might know. I’m studying for my country’s biochem entrance exams and I’m looking at old exams and I’m stuck on a question. I’ll try to translate accurately:

”Carbonic anhydrase catalyses the conversion of carbon dioxide and water into carbonic acid which in turn creates bicarbonate and hydrogen ions. The enzyme is quite fast, and its kcat=4,0•105s⁻¹ (reactions per second). How long does it take for the enzyme to process one substrate molecule?”

The answer is very simply written as ”t=1/kcat=1/4•105s⁻¹=2.5•10⁻⁶s=2.5 µs”

I don’t understand how this is calculated and the answer only leaves me with more questions. Can anyone help?