Molarity = moles of solute ÷ liters of solution 

So since Na2CO3 is the solute, and they give you the grams, just convert to moles: 
(5.3g Na2CO3)(1 mol Na2CO3/105.99g Na2CO3) 
= 0.0500 mol Na2CO3 

Also, the solution has to be in liters, so convert the mL to liters: 
(400.0 mL)(1 L/1000mL) 
= 0.400 L 

Now that you have all of the information, just put it all together into the formula: 
Molarity = moles of solute ÷ liters of solution 
Molarity = (0.0500 mol)/(0.400 L) 
Molarity = 0.125 mol/L or 0.125 M 
but the ans is 0.1
my answ is 3 decimal and yours is 1 how ever both r same bro(mudradesai)
the solution is 106g has 2 equivalent so 5.3 has 0.1
106 or 105.99 g
It is 0.01grams

hope it help