Manipulate[
 PieChart[{25, 25, c, c2},
  ChartLabels -> {"Descartes", "Fibonacci", "Archimedes", "Pascal"},
  ChartStyle -> {Hue[0.29`], Hue[0.46`], Hue[0.58`], Hue[0.22`]},
  PlotLabel -> "Percent of Students Who Prefer..."], {c, 0, 50}, {c2,
  0, 50 - c}]