probabilities on multiple choice questions I am setting a series of multiple choice questions. I have a range of option answers from 2 to 7. Some questions only have a single correct answer and others have more. 
I can tell the person taking the test what number are correct, or I can just tell them to 'select all that are correct'.
Is there a difference in the probability of someone guessing correctly between the two scenarios?
I am, by the way, absolutely clueless at maths so please dumb your answer down to the lowest common denominator! eg Yes- if you tell them there are two correct answers it is xx:yy and if you don't tell them it is aa:bb.
Thank you.

Suppose you tell them just two are correct and they guess two at random. There are $10$ ways to choose two from five (the number is small enough so you can just count them - no fancy math) and just one of the ten is right, so the probability of a correct guess is $1/10$.
If you want to count getting one or two of the two right the calculation is harder; I don't think that's what you're asking.
If you don't tell them two are right then you have to make an assumption about their guessing strategy. If they think there might be anywhere from one to five correct answers they will have $2^5 -1 = 31$ possible guesses one of which will be right, for a probability of $1/31$.
If I were taking the test I might think two or three right was the answer and would limit my guesses appropriately.
In any case giving the test taker the information will increase the probability that a guess will be correct.
