Solubility curve for sodium tetraborate in water
The solubility curve of borax is taken from here.



We can infer that solubility of borax at 100°C is around 520g/L. It is observed that borax in water exhibits supersaturation . When the solution is at 100°C and then you drop at 40°C, then a clear solution is obtained when amount of borax is 60 grams per litre. More than that, there is a tendency of the solution to overflow. See ref.1 for the more detailed explanation of supersaturation and ref.2 for the dissociation reactions happening in water. First the tetraborate ion is formed and it is a weak base in solution and thus reacts with additional water to form boric acid, a weak acid and hydroxide ion.

$$\ce{[Na2B4O5(OH)4].8H2O &lt;=&gt; 2Na+ + B4O5(OH)4^{2-} + 8H2O}$$

$$\ce{B4O5(OH)4^{2-} + 5H2O &lt;=&gt; 4H3BO3 + 2H2O}$$

Reference


https://www.enotes.com/homework-help/how-much-borax-should-dissolve-hot-water-order-721345
https://www.google.co.in/url?sa=t&amp;source=web&amp;rct=j&amp;url=https://www.smc.edu/AcademicPrograms/PhysicalSciences/Documents/Chemistry_12_Experiments/Solubility%2520and%2520Borax.pdf&amp;ved=2ahUKEwjr5b6W1tXdAhWRbisKHb1rBUAQFjAXegQIBBAB&amp;usg=AOvVaw3c9IA0jqEROPS534lSedA8