Which equals 29^2 x .01 => ((28 x 30) +1) X .01
=> (840 + 1) x .01 => 8.41

28 x 30 is an easy mental calculation, as is adding 1, as is moving the decimal place over 2 places. I am teaching this to 4th graders, in two weeks. % to decimal is next week. They can square 2-digit numbers in their heads, already.