Taking Tony's idea one step further, maybe we can work in the decimal values - I think its the right idea that we are dealing with single characters so I'm guessing ascii 188-190 are the fractional symbols (just ¼, ½, ¾)
=LEFT(A1,LEN(A1)-1)+LOOKUP(CODE(RIGHT(A1,1)),{0,188,189,190,65536},{0,0.25,0.5,0.75,0})
But since it would fail on a single character (1-9),
=IF(LEN(A1)<2,A1+0,LEFT(A1,LEN(A1)-1)+LOOKUP(CODE(RIGHT(A1,1)),{0,188,189,190,65536},{0,0.25,0.5,0.75,0}))
¿Perhaps someone can find a more elegant formula? Edit: note, also fails if the cell value is just ¼, ½, or ¾ by itself