A bag contains purple marbles and blue marbles, 29 in total. The number of purple marbles is 1 less than 2 times the number of blue marbles. How many purple marbles are there?
Lets say the number of blue marbles in the bag is X. That would make the number of purple marbles 2X-1. Combined, that makes 3X-1.
Now 3X-1=29. If we add 1 to each side we get 3X=30. Divide both sides to get X=10. Now we said that the number of purple marbles is 2X-1. Substitue in 10 to get that there are
19 purple marbles in the bag.