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One cubic foot of concrete contains the amounts for 1:2:4 and 1:2.5:5. Maybe that will help.

Sorry for the omission.

"1125 ft³ of concrete is needed. 4/5 must be 1:2:4 concrete"

4/5 of 1125 ft³ = 900 ft³

You need 900 ft³ of 1:2:4 concrete.

0.22 ft³ cement, 0.44 ft³ sand, and 0.88 ft³ gravel is used to make 1 ft³ concrete.

900 ft³ concrete × 0.22 ft³ cement/(ft³ concrete) = 198 ft³ cement

198 ft³ cement is required

The sand:cement ratio is 2:1.

2 × cement = 2×198 ft³ = 396 ft³ sand

396 ft³ sand is required

The gravel:cement ratio is 4:1.

4 × cement = 4×198 = 792 ft³

792 ft³ gravel is required.

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