For slab of 100 sq.ft having thickness 4" @ design mix M20 (1:1.5:3) means ( if we take 1 basket(doko) cement and 1.5 generally 2 basket(doko) sand then 3 basket(doko) aggregates should be mixed). [ 50kg = 1 sack ]
This calculation only shows the approximate value, cement quantity may vary.