Question

Bob the builder is making 480 kg of concrete mix.

Answer 1

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:

$1+3+4=8$

Divide the amount of concrete by this number:

$480\text{ kg} \div8=60\text{ kg}$

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:

$1:3:4$

The middle number, 3, represents the amount of sand. Multiply this with 60 kg to get the amount of sand needed.

$3 \times 60 \text{ kg} = 180 \text{ kg}$

180 kilograms of sand are needed. Let me know if you need any clarifications, thanks! (◕‿◕)

~ Padoru