Its easy WoW. All you need to know is that the probability of independent events (ie events that are unconnected like tossing a coin multiple times or in this case picking sweets out of a bag) is found by multiplying the probability of the individual events occurring together.

Lets start easy with tossing a coin. What's the probability of getting a head ?

Well a coin has two sides only one of which is a head so the probability (p) of flipping it and getting a head is 1/number of possible outcomes (n)

This is a 1/2 since a coin can only end up heads or tail ie n =2

So we know there are 6 orange sweets in the bag so whats the probability of sticking your hand in and pulling out an orange sweet ? Just like tossing a coin it's the number of picks that give us the desired outcome divided by the total number of possible outcomes. So:

Now we have our first orange sweet what's the probability of a pulling a second out. Well there are now only 5 orange sweets left and one less sweet total in the bag. So:

So what's the probability of pulling two orange sweets out as the first two picks ? Remember we just need to multiply them together:

This can be simplified by multiplying the two fractions:

Now we have been told that the probability of getting two orange sweets is 1/3 so replace p with 1/3:

Nearly there ! Mutiply both sides by three:

Mutiply both sides by n^2-n:

Subtract 90 from both sides:

Job done !

And its also then obvious that n =10.