The probability of getting all 5 right would be 1/3125
This is because each question has one choice. Making it a 1/5 chance for each question.
Now if you combine if you add the rest you will have to calculate.
You would do the calculation by multiple the chance for a correct answer for each question together.
1/5 x 1/5 x 1/5 x 1/5 x 1/5 = 1/3125
I hope this helped!