HOW to estimate problems
http://www.mathsisfun.com/numbers/estimation.html
http://www.sssoftware.com/docs/wnadoc/estimate.html
http://mathandreadinghelp.org/how_to_estimate_a_math_problem.html
http://www.sssoftware.com/docs/wnadoc/estimate.html
http://mathandreadinghelp.org/how_to_estimate_a_math_problem.html