Not very good at explanations but I'll give it a try:
Let z, d and m replace the names
From the question, we get:
z = d + 5 Equation One
d = m + 1.5 Equation Two
z + d + m = 13.1 Equation Three
We want to find out how much Maria has, so every equation has to include "m".
We can replace the "d" in Equation One with its equivalent "m + 1.5", so equation one now reads: z = m + 1.5 + 5
Now we know that "m" is in each of the equations, we can replace it in Equation Three, so:
m + 1.5 + 5 + m + 1.5 + m
z + d + m
If you work that out, it comes to:
3m + 8 = 13.1 which can be solved like a normal equation:
3m + 8 = 13.1
3m = 5.1 (minus
m = 1.7 (divide by 3)
So the answer is Maria has Â£1.70. (Of course, if you wanted to know how much the others had, just work it out some more)
Rather long winded, but gets to the point.
Hope it helps!