Steven subtracts the units digit from the tens digit for each two-digit number. He then finds the sum of all his answers.  What is the value of Steven\textquotesingle s sum? 
Overseas Competition->Knowledge Point->Calculation Modules->Sequences and Number Tables->Arithmetic Sequences
The sum of all the tens digits is $$\left(1+2+3+4+5 +6 +7+8+9\right)\times10$$.  The sum of all the units digits is $$\left(0 + 1+2+3 +4+5 +6 +7+8+9\right)\times9$$.  Therefore Steven\textquotesingle s sum is $$\left(1+2+3 +4+5+6+7+8+9\right)\times1=45$$. 