can someone tell me how to work this question out? need to know for exam. A drink stall sells small,medium and large cups of fruit drink for...

Let S, M and L stand for small, medium and
large. 

So S(1.5) + M(2) + L(2.5) = 1360.  This means each
size cup times its price, all added together equals the entire amount of
$1360.

 But you can't solve with three different variables,
so you have to get everything so you only have one variable.  Let's see what you
know:

3S=M    3 times as many medium drinks as small or,
turning it around, 1/3M=S.

L= M-140    Number of large cups
is 140 less than the number of medium

Now, plugging in: 
1/3M(1.5) + M(2) + (M-140)(2.5) = 1360.

By working out the
value of M mathematically, you will know how many medium cups were sold.  1/3 of that
number will be the number of small cups, and the number of Medium - 140 will give you
the number of large cups.