Mayflies can live at the bottom of lakes for 2 to 3 years before they become winged adults. mayflies are between 4/10 inches and 1.6 inches long. if this mayfly is 4/10 of an inch long, how can you write 4/10 in simplest form.

To simplify any fraction, look for a common factor in the top number
and the bottom number.  If you find one, divide the top and bottom
both by their common factor.  Eventually, you reach the point where
their only common factor is ' 1 ' ... that's when it's in simplest form.

4  and  10  both have  2  as a factor. 

When you divide top and bottom both by  2,  you have  2/5 .
This is the simplest form of  4/10 .

None of this has anything to do with mayflies.  You can use the same
process to simplify fractions in ANY situation where fractions arise.