Write three ratios that are equilvalent to the one given:18 right-handed students for every 4 left handed students

Well, your ratio given is 18 to 4. so multiply both numbers (18 and 4) by some number and you will have an equivalent ratio.

example: if you had 2 times as many people, then an equivalent ratio would be 18*2 to 4*2 = 36 to 8