Naomi’s fish is 40 mm long her guinea pig is 25 cm long how much longer is her guinea pig than her fish

1.) Convert the measurements of Naomi's pets so that the two has the same unit of measurements. I will use cm.
                     1 mm = .01 cm;
Therefore:  40 mm = 4 cm is the length of the fish
2.) Subtract the pets' measurements:
         25 cm - 4 cm = 21 cm
3.) Therefore, Naomi's guinea pig is 21 cm longer than her fish.