Let n represent the number.
.. (3/5)n - 1 = 23 ... three-fifths of a number decreased by one is twenty-three.
Please note that the same English description might be interpreted as
.. (3/5)(n-1) = 23.
We'll use the first interpretation, as it results in the number being an integer.
.. (3/5)n = 24 ... add 1
.. n = (5/3)*24 ... multiply by 5/3
.. n = 40 ... carry out the multiplication
The number is 40.