Three cascaded modulus-5 counters have an overall modulus of ________.

ANS:C - 125

Simply each cascaded state consider f/mod for frequency division.

And the next stage takes f as previously divided frequency.
Ex-100/5=20 (stage1) ,
20/5=4 (stage2) ,
4/5=0. 8 (stage3) or 100/125=0.8 here 125 is 5*5*5.