There are 20 players in a table tennis competition. each player must play exactly one match against every other player. how many matches are there in the competition?

Empty box + box + empty box fill in the box is equal to 30 how

Ineed in figuring out this problem.

Luke started a weight-loss program. the first week, he lost x pounds. the second week, he lost pounds less than times the pounds he lost the first week. the third week, he lost 1 pound more than of the pounds he lost the first week. liam started a weight-loss program when luke did. the first week, he lost 1 pound less than times the pounds luke lost the first week. the second week, he lost 4 pounds less than times the pounds luke lost the first week. the third week, he lost pound more than times the pounds luke lost the first week.