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We intend to attack those numbers the sum of whose unit digits give 10.
We can write the expression given as
( 1 + 9 ) + ( 1 8 + 2 ) + ( 3 + 1 7 ) + ( 5 + 1 5 ) + ( 2 + 8 )
= 1 0 + 2 0 + 2 0 + 2 0 + 1 0
= 8 0