Answer:
Assuming that the longest blue whale on record was about 18 scuba divers in length and a scuba diver is 6 feet length, we want to estimate the length of the whale.
Length of the whale = 18*6
You can multiply mentally by using compensation. Substitute a number for 18 that is easy to multiply.
20*6 = 120
Now adjust. Subtract 2 groups of 6, or 12.
120 - 12 = 108
and that is the length of the whale in feet