1/2 chance to get a dinosaurs sticker for the first bag. 1/6 chance to get a dinosaurs sticker for the second bag.
1 = 1 x 10 exponent 0
27.5 = 2.75 x 10 exponent 1
Let us always base on the present age. We denote this by x. Now, the age he had 2 years ago would then be denoted as (x-2). Let's equate this to 20 years old.
x - 2 = 20
x = 20 + 2
x = 22 years old
He should be 22 years old now.
Let's check the other condition. After 1 year, his age should be (x+1). Let's equate this to 23 years old.
x + 1 = 23
x = 23 - 1
x = 22
Thus, this is possible if Reuben is 22 years old as of the present.
There's only information to answer question C.
We can only know that 32 marbles were chosen. (yellow and black)
Why?
To answer the first two questions (a and b), we need to know the total number of marbles, so, we can only answer the last question (c).
We know that 7/10 of the 20 yellow marbles were chosen.
So, calculating we have:

Also, we know that 18 black marbles were chosen, so, the total of marbles chosen is equal to 32 (yellow marbles and black marbles)

Have a nice day!