1.15 just need a little more thought. You can transform the equation to $2\times 10^a = 19N + 4$.  Think more carefully what this equation means. Hint: what do you do if you divide 200 by 19?  2000 by 19?  20000 by 19?

1.16 is correct.  Pairing is a good method that can be applied to many problems.  Good job!