Nice problem. I assume that in all three cases the initial grass volume on the pasture is the same. Let this volume be p units (for "pasture"). Assume also that the grass grows g units per day, and 100 sheep eat s units of grass per day. The problem statement gives two equations in these three variables. Note however that the answer (the number of days corresponding to 1 hectosheep ) can be expressed as a function of g / p and s / p, and the two equations above can be rewritten as equations only in g / p and s / p. Thus, we have two equations and two unknowns.