Math Marble brain teaser

So today In math for our brain teaser we had to watch a video of a man putting marbles in a cup he told us how many he put in at a time we had to try and work out how many marbles he put in the cup all to together first he put 10 marbles then 20 marbles then 30 marbles then lastly 40 marbles all that counted 100.I thought it was easy because I new 10 marbles +20 marbles = 30 then I new 30 marbles + 30 marbles = 60 marbles the 60 marbles + 40 marbles = 100