There are 4 bags, and each bag contains 3 blue marbles and 2 yellow marbles. How many marbles are there in total?

Each bag contains 3 blue marbles and 2 yellow marbles.

$$3 \text{ blue marbles} + 2 \text{ yellow marbles} = 5 \text{ marbles per bag}$$

There are 4 bags, and each bag has 5 marbles.

$$4 \text{ bags} \times 5 \text{ marbles/bag} = 20 \text{ marbles}$$

The total number of marbles is $\text{20 marbles}$.