The volume of water to be added to convert 10 ml of decamolar HCl solution to decinormal solution is 

Milli moles of concentrated HCl = milli moles of diluted  HCl

10 × 10 = V × 0.1 

 V = 1000ml 

Thus 990 ml of water should be added to 10 ml of concentrated HCl to get decinormal solution.