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Assume water/cement ratio of 0.45 BY WEIGHT for a slab mix, so the water volume will be about 1.4 parts by volume, or 1.4 water: 1 cement: 2 sand: 3 coarse aggregates, 7.4 parts total.

For 0.15 m^3, you need about 0.15/7.4 or about 0.021 m^3 of cement. That will be about 66 KG of cement, or a little more than one 50-kg sack (a common size). The US standard bag is 94 lbs (43 KG), you would still need two bags.

Note that you may wish to adjust the cement and sand fraction based on the nature of the coarse aggregate. If the coarse aggregate is very angular and relatively uniform in size, it can be very difficult to work with especially if you are mixing by hand. Under those circumstances you might consider 1 cement: 2.5 sand: 2.5 coarse aggregate or even 1:2:2.

Dave