It is as easy as determining the slope of the line from the standard curve and using it in the formula: OD=ec.
So, using this method,
the kids’ cordial had an OD of 0.8.
the slope of the line is 5
0.8 = .05c so
c=0.8/.05 = 16 % (v/v)
…rather than the 20% (v/v) recommended by the manufacturer. This is almost the identical result as when we read it off the graph.