True thickness: limestone vertical 600 m

Principle: the vertical thickness $t_v$ (as in a vertical hole) is converted to the true thickness (perpendicular to bedding) by

$$t = t_v\cos\delta$$

where $\delta$ is the dip of the bed. The strike (N75°E) and dip direction (NW) do not affect the thickness; only the dip amount, $50^\circ$, is needed.

Calculation

$$\begin{aligned} t &= 600\cos 50^\circ \\ &= 600 \times 0.6428 \\ &= 385.67\ \text{m} \end{aligned}$$

Answer: true thickness of the limestone = 385.7 m.

Volume under 550 km² (for reserve)

$$V = 550\ \text{km}^2 \times 0.38567\ \text{km} = 212.1\ \text{km}^3 \approx 2.12\times 10^{11}\ \text{m}^3$$

(taking the 550 km² as the area measured on the bed).