The true thickness (t, in m) of bed B in the given diagram is ..... 
Given:
Concept:
Looking at the diagram:
Relationship:
$$t = \text{Horizontal width} \times \sin(\text{dip angle})$$
Calculation:
$$t = 20 \times \sin(30°)$$
$$t = 20 \times 0.5$$
$$t = 10 \text{ m}$$
Answer: 10 m
