The new solution will be 80% paint and 20% thinner.
Here's how to calculate it:
Initially, we have 20 litres of solution, out of which 85% is paint. So, the amount of paint is:
20 litres x 85% = 17 litres
And the amount of thinner is:
20 litres x 15% = 3 litres
Now, if we add 5 litres of thinner, the total amount of thinner becomes 3 + 5 = 8 litres.
The total volume of the solution is still 20 litres + 5 litres = 25 litres.
Hence, the new solution is made up of 17 litres of paint and 8 litres of thinner. Therefore, the percentage of paint in the new solution is:
(17 litres / 25 litres) x 100% = 68% paint
So, the new solution is 68% paint and 32% thinner.