"clear as ditch-water"- means that a text, an explanation, an idea is not understandable; „ясно като в мъгла"

Explanation:  When I was a child the fractions were clear as ditch-water form me until my grandfather explained them to me.

Когато бях дете дробите ми бяха за мен мъгла, докато дядо ми не ми ги обясни.