The number 25 in decimal (base ten) can be converted to binary (base two) as follows:

1. Divide the number by 2.
2. Write down the remainder.
3. Continue dividing by 2 until you get a quotient of zero.
4. The binary equivalent is the remainders, written in reverse order.

Let's do this for 25:

1. 25 divided by 2 is 12 remainder 1.
2. 12 divided by 2 is 6 remainder 0.
3. 6 divided by 2 is 3 remainder 0.
4. 3 divided by 2 is 1 remainder 1.
5. 1 divided by 2 is 0 remainder 1.

So, writing the remainders in reverse order, we get 11001.