A bag contains 3 blue marbles and 2 yellow marbles. If there are 4 such bags, what is the total number of marbles?

Each bag has 3 blue marbles and 2 yellow marbles.

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

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

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

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