其它
Elsa started to raise some chicken in her farm in January. The number of chicken triples every month, while she goes to the market to sell $81$ chicken every month. On April when she came back from the market, she found she had no chicken left.  How many chicken did she raise in January? 
Overseas Competition->Knowledge Point->Word Problem Modules->Inverse Operation Problems
$81 \div 3 = 27$  $(81 + 27) \div 3 = 36$  $(36 + 81) \div 3 = 39$  $(39 + 81) \div 3 = 40$ 