I've used this formula to calculate the solution.
The number of diagonals of a polygon is $(n(n-3))/2$, where $n$ is the number of sides of a polygon.
But I'm getting answer as decimal point. Is the question wrong or am I doing any calculation mistake?
Obviously the question is worded wrongly. In any polygon, the number of diagonals is one less than a triangular number, and 60+1=61 is not triangular.
In three dimensions, a regular dodecahedron has 60 face diagonals, plus 100 internal (body) diagonals.