Concept: (i) Perimeter of a triangle: The sum of all three sides of the triangle is called its perimeter.
(ii) Fact: The sum of any two sides of a triangle is always greater than its third side.
Given: First side of triangle = 5.6 unit.
Second side of triangle = 19.7 unit.
Let third side of the triangle be 's' unit.
The perimeter of triangle = (5.6 +19.7 + s) unit.
But as per the fact given above, s < (5.6 +19.7)
or, s < 25.3 -------------(i)
Now, we shall add 25.3 both the sides in equation (i) to make the left hand side equal to the perimeter.
So, the perimeter of triangle = s + 25.3 < 25.3 + 25.3 = 50.6
Now, to make 50.6 as next whole number smaller than the perimeter which will be 50.
Hence, 50 is the biggest whole number smaller than the perimeter of the triangle with a side of length 5.6 unit and a side of length 19.7 unit.