The ratio of detergent to water in the washing solution is 1:4. So if Li Ting needs 5 litres of washing solution, there must be 1 litre of detergent and 4 litres of water.

The detergent bottles come in 500ml, so she will buy 1000ml / 500ml = 2 bottles of detergent.

If she uses the 1 litre of detergent to make 10 litres of washing solution instead, the other 9 litres has to be water.

So the new mixing ratio used is 1:9.