Sum

 

Find the last two digits of the following sum: 5 + 10 + 15 + ... + 100.     

 

S  =  ( 20 / 2 )( 5 + 100 )     

S  =  10 • 105     

S  =  1050                           so the last two digits are 50     

 

The formula for the sum of a finite arithmetic series is S = (n/2)(a₁ + a_n),     

where 'n' is the number of terms, 'a₁' is the first term, and 'a_n' is the last term.