Some cows live on a grassland. $10$ cows can eat all grass in $9$ days. $12$ cows can eat all grass in $7$ days. The amount of new grass that grows each day is constant. $24$ cows can eat all grass in~\uline{~~~~~~~~~~}~days. 
Overseas Competition->Knowledge Point->Word Problem Modules->Newton's Problem of Cows and Fields
Suppose a cow can eat $1$ m\textsuperscript{2}~of grass each day.  The amount of new grass that grows each day: $(9\times10-12\times7)\div(9-7)=3$ m\textsuperscript{2}.  The amount of grass originally: $$90-9\times 3=63$$ m\textsuperscript{2}.  $$24$$ cows can eat all grass in $$63\div \left( 24\times1-3 \right)=3$$ days. 