I just wanted to clarify some more logic points. The reason, other than it just, you know, making sense, that the conclusion was drawn that beyond 60ft the symbol could be visible but not legible, and not the other way around (legible but not visible) is there is a second proposition that is really more implicit than explicit.
We've got...
X: Symbol is visible.
Y: Symbol is legible.
and..
P1: If and only if distance<=60, then X&Y=True
P2: Y implies X
converse of P1: if distance >60, (X&Y)=false
equivalent statement: if distance >60, X=false, or Y=false
equivalent statement of P2: !X -> !Y
so... the truth table for distance>60 is
x y
1 f f
2 t f
I'm not sure if I made that clear. In any case, assuming something must be visible to be legible, and beyond 60 feet, the thing is not legible AND visible, then the conclusion is that beyond 60 feet, the object is not legible, but may or may not be visible. savvy?