\tWhen there is a decrease in pressure, the equilibrium will shift towards the side of the reaction with more moles of gas.explanation

@ jatin 
the statesment are very simple . consider an example – 
suppose the reaction , are in equilibrium , that means the amount of reactant , = amount of product or the rate of recatant converting into product = the product converting into reactant . 
so, if u will increase the pressure in the reactant side , then the equilibrium will gng to shift in that direction in which it can use that increased pressure into some usefull work , as it doesnot required extra pressure . 
so, it will shift into the more moles of gas side as it requires pressure and also , the pressure 
can be used here efficiently and equilibrium will be maintained as  before . 
HOPE IT CLEARS 
ALL THE BEST ..