Strike when dip direction is N40°W

The strike is the direction of the horizontal line on the bed and it is always at 90° to the dip direction.

Given: Dip direction = N40°W, azimuth = 320°.

$$\begin{aligned} \text{Strike} &= \text{dip direction} \pm 90° \\ &= 320° - 90° = 230° \ \text{or}\ 320° + 90° = 50° \end{aligned}$$

These two azimuths (50° and 230°) are the two ends of one line, so the strike is N50°E (S50°W).

                    N
     dip N40W \     |     / N50E
                \   |   /
                  \ | /
  W ----------------+--------------------- E
                  / |
                /   |
         S50W /     |
                    S
   ( / = strike line N50E-S50W,  \ = dip direction N40W, at 90 deg to strike )

Answer: Strike = N50°E (S50°W).