Ateacher grades 25 students essays in 4 hours. assuming he grades at the same speed, how long will it take him to grade 35 essays? what is the constant of this variation?

For this case, the first thing we must do is define a variable:
 x: number of hours
 y: number of trials
 We write the equation that represents the problem:
 y = (25/4) x
 For 35 essays we have:
 35 = (25/4) x
 Clearing x:
 x = 5.6 hours
 
 it will take him to grade 35 essays about: 
 x = 5.6 hours
 the constant of this variation is:
 25/4 essays / hour