Bob the builder is making 480kg of concrete mix

To answer this item, we are first to determine the common factor between the amounts of the cement, sand, and gravel.

If we let x be this factor, the amount of the cement would be x. Similarly, the amount of sand is 3x, and lastly the amount of gravel is 4x. Then, we establish the equation that would let us relate the amounts.

x + 3x + 4x = 480

Simplifying,

8x = 480

x = 60

Hence, the amount of cement is 60 kg, that of sand is 180 kg. Lastly, the amount of gravel is 240 kg.