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This problem trains for: SAT-I, AMC-8, GMAT, AMC-10.

Fifteen slugs participate in a race. They wear labels from 1 to 15. During the race, one of the slugs gets hungry and decides to quit the race and go to a restaurant. The sum of the labels of the slugs who finished the race is a number that ends in 0. What was the label of the slug who dropped out?

The sum total of the participating labels is given by the sum:

equation

The only positive number smaller than 15 that subtracted from 90 gives a difference that ends in 0 is 10.