I realise that in my earlier post it may have been too
much to read so I have tried to simplify it:

Suppose I have the following set-up

UNAVAILABILITY
A B C D E F ....
1 Unit Amount 1-Feb 2-Feb 3-Feb 4-Feb ....
2 Maths £10 a
3 English £11 a ab b
4 Music £8 abc b abcde
5 P.E. £9
6 Geography £6 a
7 History £6
8 Drama £7
9 Science £12 a
... .
... .
... .

Now suppose that I want to add up the amount that is
attributed to "a" i.e:
£10 + £11 + £11 + £8 + £8 + £6 + £12 = £66.

Similarly, the total amount attributed to "b" i.e:

£11 + £11 + £8 + £8 + £8 = £46.

"c": £8 + £8 = £16.

"d": £8.

"e": £8.

How would I do this?

Can I use a for.. from...do loop?