Exercise 4.4.20

[HamzaIbrahimi]

Have an exam so I'm just going through some exercises. But for the proof the simplification of the expression should be:

$n + n + 1 + n + 2 = 3n + 3$

It seems the answer provided, $3n + 6$, is incorrect.

[spamegg1]

Thanks for reporting! I'll fix it.

[spamegg1]

Fixed in #21