Explain why salt water and sugar water have a lower melting point than water.

This is the same reason why sea water and sugar solution have higher bp than pure water. If the solute is non-volatile there is a greater intramolecular attraction between solute and solvent. Thus it is quite more difficult to melt impure ice. Generally the formula for the change in mp for water is:

delta Mp = fpc x molality where fpc is knows as cryoscopic constant