The 3 significant figures are the figures that best represent the number. In 15.68, the number 15 is important because it’s close to the complete number, but that’s only 2 figures, so what should the decimal be? 0.68 is closer to 0.70 than it is to 0.60 and we have to use only one figure, so we pick the closer one, which is 0.70 and ignore the zero because that would make 4 figures. So to 3 significant figures 15.68 is 15.7.