This is not under stocks

Concrete class 20, recommended for most light structural works. 1:2:4 class.

1:cement
2:Sand/Fine aggregate
4:Coarse aggregate/ballast(aggregate should not exceed 20mm)

Water cement ratio is 0.5.
The fundis can sometimes skip a component in the batch. Hope you had someone check your reinforcement if they were properly placed.

Good luck.

The water cement ratio is a guide, depending on the casting conditions. Google hydration of cement. Its not strictly 0.5, the method of batching might necessitate more water than that. Its more of a text book thing/ideal condition. Make sure the water is not too much but enough to let the concrete be moved(workability)

You mean u are doing it yourself? No senior fundi, approved arch & structural maps?
Since they contain all this info.

Class 15 the 1:3:6 is for blinding, the concrete that is first laid on the ground/soil. Acts like a protection.
1:1.5:3 is for class 25. Ask him what class he used in his design, if he used class 25 then kindly follow his instructions. I find class 25 to be on the high side for a residential building unless you plan on doing some light industrial work on the suspended floor.

Just do what the engineer is telling you, especially if he is the one who did the the structural design.
The one he is Class 25 (i.e 1:1.5:3).
NOTE: These ratio are in terms of weight and not volume, so, the one of fundi sounds 'funny' to me, but i assumed his is volume based.
If nothing was specified in the approved design/construction drawings or the BoQ specific item, then call the design engineer and confirm.