So, the ration is 17:3, you know that. Therefore, the change from # of students to the 17 is the same change as # teachers to 3. Knowing this, you also know that the total # of people ihas the same change as the total ratio. So 17+3 = 20. 740/20 = 37. So the change is 37. 3x37= 111 17x37=629
You can check to make sure this answer is correct by adding the two final numbers. 111+629 = 740. (Some find it easier to see lined up really pretty)
17 -- 3 total 20
x -- y total 740
Or, if you are visual, look at the picture I drew:
