Bob the builder is making 480kg of concrete mix. concrete is made by mixing cement, sand and gravel in the ratio 1: 3: 4

The amount of sand builders need is 180 kg.

Step-by-step explanation:

It is given that the Bob the builder is making 480 kg on concrete mix.  Concrete is made by mixing cement, sand and gravel in the ratio 1 : 3 : 4.

Let the amount of cement, sand and gravel is x, 3x and 4x respectively.

Since Bob the builder is making 480 kg on concrete mix, therefore

Divide both sides by 8.

The value of x is 60.

The amount of sand is 3x.

Therefore the amount of sand builders need is 180 kg.