We know that 1/7 + 2/7 + 4/7 equals 7/7 = 1
The mixture is 1:2:4. The total is 7 parts. The cement is 1/7, sand is 2/7, and gravel is 4/7.
1/7 = 0.142 cement.
2/7 = 0.285 sand.
4/7 = 0.571 gravel.
Would these calculations be used in any way ?
The calculation of the VOLUME amounts is done in the manner shown by timur WHICH USES THE AIR SPACE AND THE RATIO IN THE FORMULA.
Those calculations allow the builder to determine the volume of concrete which will be poured if a mix of 1:2:4 is used and the ingredients are delivered in the sizes of containers containing the amounts of free space (air) that the question states.
GO BACK AND READ THE QUESTION THAT YOU POSTED ESPECIALLY THE BITS ABOUT X,Y,Z AND THE AIR COMPONENT
YOU WERE GIVEN THE EQUATION TO SOLVE.
Coincidentally, I'm thinking of adding a driveway.