"to make concrete is takes 8 parts crushed gravel, 7 parts water and 5 parts cement mix. a worker needs to make 220kg of concrete. how much crushed gravel, water and cement mix is needed? "
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Ratio of crushed gravel to water to cement mix = 8:7:5 (for a total of 20 parts)

Thus . .
8/20 = fraction of crushed gravel
7/20 = fraction of water
5/20 = fraction of cement mix

If total is 200 kg, then . . .
(8/20)*(200 kg) =  80 kg crushed gravel
(7/20)*(200 kg) =  70 kg water
(5/20)*(200 kg) =  50 kg cement mix

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