The necessary & sufficient condition for the differential equation of the form $$\,\,M\left( {x,y} \right)dx + N\left( {x,y} \right)dy = 0\,\,$$ to be exact is

The differential equation $${{{d^4}y} \over {d{x^4}}} + P{{{d^2}y} \over {d{x^2}}} + ky = 0\,\,$$ is

The void ratio of a soil sample is 1. The corresponding porosity of the sample is _______.

Grading of aggregate in a concrete mix is necessary to achieve