The sides of a triangular plot are in the ratio of 3:5:7 and its perimeter is 300 m.Find its area 

let the sides be 3x,5x and 7x
then 3x+5x+7x=300
or 15x=300
or x=20
so 
all three sides are
3x=60
5x=100
7x=140
then the area by herons formula will be
  (s)(s-60)(s-100)(s-140) under root
= (150)(90)(50)(10) under root                                                             since s=perimeter/2
=1500*4500 under root
=6750000 under root
=2598.07 m²