How much fuel will a Cessna 172 hold?

56 US gallons

I have a model 172K 1969 manufacture Cessna Skyhawk. It has 38 Gallons usable and 4 gallons unusable therefore the fuel total it will hold is 38+4=42 Gallons.

The 56 Gallon answer isn't clear about which model it is and they do vary by model.