The area of a rectangle is 40 square meters,what is its length and width when the width is thrice its length?

Suppose L represents the length and W represents the width. Relevant equations are
  L*W = 40 m^2
  W = 3*L
Substituting the second into the first, we get
  L *(3*L) = 40 m^2
  L^2 = (40/3) m^2    (divide by 3)
  L = sqrt(40/3) m    (take the square root)
  L ~ 3.651 m
  W = 3*L = sqrt(120) m ~ 10.954 m

Oddman is correct, as usual.