It's quite possible that the question includes some data about the sensitivity of the melting and boiling points to the proportion of glycol, then use them, keeping an eye on molar versus mass versus volume proportions.
Though, these relations are far from linear. Outside education, the proper method would be to look for experimental curves, which are plentiful in this case. Also keep in mind, beyond the present question, that volumes are not additive, especially with alcohols and water, and that propylene glycol replaces often the toxic ethylene glycol.
Boiling: the question probably wants you to answer that, but glycol also increases the viscosity, which is important to lubricate the pumps. Car coolant fluids contain more additives, especially against corrosion, possibly for wetting, against foam, and so on.