The crocodile grows 1/5 the length he was the last week.
SO, you need to find a number that will ensure that the end result is 30.
What number plus 1/5 of it equals 30?
25.
25/5= 5
25+5= 30
So, the number is 25. The crocodile was 25 cm long last week.
I hope this helps!
~kaikers