Students and Calculators:

One of the most prevalent problems amongst my students has been the lack of numeracy. 
Wikipedia defines numeracy as 
"the ability to reason and to apply simple numerical concepts"

Wikipedia then explains that
"Substantial aspects of numeracy also include number sense, operation sense, computation, measurement, geometry, probability and statistics. A numerically literate person can manage and respond to the mathematical demands of life".   
 
 See http://en.wikipedia.org/wiki/Numeracy
In my opinion, excessive reliance on modern calculators prevents the learning of, or degrades existing ability in number sense, operation sense and computationReferring again to Wikipedia

"Mathematics" is the abstract study of topics encompassing quantity, structure, space, change, and other properties; it has no generally accepted definition."

http://en.wikipedia.org/wiki/Mathematics

I understand this as meaning that mathematics is ultimately about understanding our world as it is measured in numbers.  Number Theory is a major branch of advanced mathematics.    

You may expect me to discourage your use of your calculator, except when it is necessary.

Write down (without using a calculator) the value of

 ((150)/3) - 1
-------------------------------         (= 7). 
(20-17) + 4

If you agree you may be numerate.