The average density of porridge with butter is 2 times more than oil and three times less than just porridge.

The average density of porridge with butter is 2 times more than oil and three times less than just porridge. What is the ratio of the mass of porridge to the mass of butter?

Rк, Rм, Rc – porridge density, oil density and average density, respectively.
By condition:
Rc = 2Rm;
Rc = Rk / 3;
2Rm = Rk / 3;
Rk = 6Rm;
The density of the oil is six times less than the density of porridge.
The average density is equal to the sum of the masses divided by the sum of the volumes:
(Mk + Mm) / (Vk + Vm) = Rc;
We will only work with the density of porridge:
(Mk + Mm) / (Vk + Vm) = 2 Rm (1);
Vk = Mk / Rk = Mk / 6Rm (2);
Vm = Mv / Rm (3);
Substitute (2) and (3) in (1):
(Mk + Mm) / (Mk / 6Rm + Mm / Rm) = 2 Rm;
(Mk + Mm) / ((Mk + 6Mm) / 6Rm) = 2 Rm;
6Rm * (Mk + Mm) = 2Rm * (Mk + 6Mm);
3 (Mk + Mm) = Mk + 6mm;
3 Mk + 3 Mm = Mk + 6 Mm;
2 MK = 3 mm;
MK = 1.5 mm;
Answer: The mass of porridge is one and a half times the mass of oil.