Math Is Fun Forum

I am currently in university, busy with an encryption project, more specifically Elliptic Curves over prime integers.
In order to double a point on the elliptic curve, one has to use the formula:

lambda = [3*(x1)^2 + a] / 2*(y1) mod p,
where x1 and y1 is a point on the curve(x1,y1), and 'a' is part of the equation(y^2 = x^3 +a*x+b mod p), and p is the prime modulus.

Below is an example I found on the Internet for computing lambda:
Location: http://www.site.uottawa.ca/~chouinar/Handout_CSI4138_ECC_2002.pdf on page 4

Point: (3,10)
a = 1
p = 23

lambda = [3*(3)^2 + 1] / [2*10] mod 23
          = 5 / 20 mod 23
          = 0.25 mod 23
          = 6 mod 23

The problem I am having is: How do they convert the decimal result(0.25) to an integer result(6).

Another example of this can be found at: http://www.certicom.com/index.php/34-quiz-2--solutions Solution Number 5.

Your help is much appreciated.

Thank you