Once children are really secure and confident with adding 9 to a 2-digit number they can quickly catch on to how to add 19, 29 etc. Once again the process is the same: add the nearest whole ten and then subtract 1. Once this has been mastered similar techniques can be used to add 2-digit numbers that have 8 in the units, subtracting two rather than one, which makes adding any 2-digit numbers fairly straightforward!
This page, and many others, can be found in our Year 4 maths worksheets under Knowing Number Facts.