The turtle is initially 20 5/6 inches below the surface.
After which he dives to a depth of 32 1/4 inches
then the initial position below - 20 5/6 inch
final position - 32 1/4 inch
the difference in the positions can be calculated taking the difference between initial and final position
this can be done with a simple subtraction of fractions
difference =

when subtracting mixed fractions the whole numbers are first subtracted
=

=

we can now bring the denominators to a common denominator which is 12
then in the first fraction 1/4, denominator is 4 , to bring it to 12 we can multiply the whole fraction by 3

next fraction 5/6 , denominator is 6, to bring it to 12 we multiply whole fraction by 2

now since 3/12 is smaller than 10/12 so we borrow a whole number from 12 outside the brackets, then

then
= (12 - 1) (

)
= 11

therefore change in turtles position is 11 5/12 inches