Set this up as an equation with a variable for turtles. Count up the other side of the scale (without turtles). Add up the three 1s and the 4 to get 7. Then add up all four 7s. This will get you with an equation 3t+4=28.
To solve this, first subtract 4 from both sides to get 3t=24. Now divide both sides by 3. 24/3=8. Each turtle weighs 8 units.
