A ship's log gives its latitude as 18° 42' 27'' N…

Step 1. Seconds into minutes. $27'' = \dfrac{27}{60}' = 0.45'$, so the angle is $18^\circ 42.45'$.

Step 2. Minutes into degrees. $42.45' = \dfrac{42.45}{60}^\circ = 0.7075^\circ$.

Step 3. Add. $18^\circ + 0.7075^\circ = 18.7075^\circ$.