F X Varies Directly As Y And X = 6 When Y = 15, Find The Value Of X When Y = 25?

When y is larger by a factor of 25/15, x will be, too.
  X = (25/15)*6
  = (5*5*2*3)/(5*3)
  = 5*2
  = 10

The value of x is 10 when y is 25.