$解：①當(dāng)?0≤x≤60?時，如圖所示$
$由?GF//CD，?得?\frac {EF}{ED}=\frac {GF}{CD}，?即?\frac {y}{60-x+y}=\frac {1.6}{4.8}?$
$∴?y=-\frac 12x+30?$
$②當(dāng)?60?＜x\leqslant 120，如圖所示$
$由?GF//CD，?得?\frac {EF}{ED}=\frac {GF}{CD}，?即?\frac y{x-60+y}=\frac {1.6}{4.8}?$
$∴?y=\frac 12x-30?$
