Stevie has some marbles. freddie has twice as many marbles as stevie. danny has 5 more marbles than freddie. in total, they have 55 marbles. how many marbles does danny have?

Let's denote the number of Stevie's marbles as x, then Freddie has 2x marbles and Danny has 2x+5. In total, they have x+2x+2x+5 marbles that is equal to 55.
x+2x+2x+5=55, then 5x+5=55, and 5x=55-5. 
5x=50, from where x=50÷5, so x=10.
Stevie has 10 marbles, Freddie has 2x=2×10=20 marbles and Danny has 2x+5=2×10+5=20+5=25 marbles.