Question: Mina has $5$ more marbles than Kristy,  

Kristy has $2$ more marbles than Seb,  

altogether they have $30$ marbles.

How many marbles do they have each?

Answer: 
Hint: Construct an equation, given $x = $ the number of marbles that Seb has.

If Seb has $x$ marbles, how many does Kristy have? $\;\;(x + \;?\;)$

And then how many marbles does Mina have? $\;\;[(x +\; ?\;) \;+ \;??]$

Add the number of marbles that Seb has plus those of Kristy, plus those of Mina, and set equal to $30$.

Then you have an equation with one variable, $x$, that you can solve.
s = # of marbles that Seb has, k = # of marbles of Kristy, and m = # of marbles of Mina. Then:

m = k + 5, k = s + 2, and so: s + s + 2 + s + 2 + 5 = 30. So 3s + 9 = 30 so 3s = 21 and s = 7.

Seb has 7 marbles, Kristy has 9 marbles, and Mina has 14 marbles.