We can take the ratio of two same quantities.
Quantities must have the volume in the same units.
We know 1 liter is equal to 1000 milliliters
We have here 3.31 liters
We need to multiply this by 1000 to convert it to milliliters.
We get
3.31 × 1000 = 3310 milliliters
So
3.31 L : 900 ml
= 3310 : 900
Dividing by 10 we get
= 331 : 9
Now the factors of 9 are 3 × 3.
But 331 is not divisible by 3
Hence this ratio cannot be simplified further.
The simplified answer is 331 : 9