i’ll express this in model format, where Y has 3 times more water than X and Z has 4 1/5 litres less than Y. then i’ll add the 4 1/5 litres to the total volume so that Z has equal volume of water as Y, meaning that Z + Y + X = 7 X because Z=Y=3X. then i’ll use the total volume of 20 3/10 to divide by 7 to find X, then i’ll find Y using X and then subtract 4 1/5 litres to find Z. i hope you understand!