妈妈的年龄为:81×4/(4+4+1)=81×4/9=36(岁)
爸爸的年龄为:81×4/(4+4+1)=81×4/9=36(岁)
小青的年龄为:81×1/(4+4+1)=81×1/9=9(岁)或者81-36-36=9(岁)