An employee makes up a compost by mixing 3 parts peat with 1 part sand...

An employee makes up a compost by mixing 3 parts peat with 1 part sand.
He mixes 24 kilograms of this compost.
How much peat does he need?

For some reason I can't figure out how to do primary school maths :-S

3 + 1 = 4
24 / 4 = 6
so
18 kg peat + 6 kg sand

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C = compost
S = sand
P = peat

C = S + P

we have 3 parts peat to 1 part sand: P = 3S

C = 3S + S = 4S (total parts).

C = 24 kilograms.

24 = 4S,

6 = S, we use 6 kilograms of sand, and 18 kilograms of peat.