The ratio given for cement to sand to gravel is 1 : 3 : 4, and all these make concrete.
Add all the numbers in the ratio:
Divide the amount of concrete by this number:
This means that each unit in the ratio represents 60 kg when there is a total of 480 kg of concrete. Now let's look at the given ratio again:
The middle number, 3, represents the amount of sand. Multiply this with 60 kg to get the amount of sand needed.
180 kilograms of sand are needed. Let me know if you need any clarifications, thanks! (◕‿◕)
~ Padoru