Spreadsheet and weighted means

[Nicolas Goaziou <n.goaziou@neuf.fr>]

Hello,

I stumbled upon this problem : I'd like to compute the weighted mean of
some values, even though cells might be empty. In fact, I'm aiming at
something like this :

|           | Coeff. |    0.2 |    0.5 |      1 |
|-----------+--------+--------+--------+--------|
| Name      |   Mean | Test 1 | Test 2 | Test 3 |
|-----------+--------+--------+--------+--------|
| Student A |     10 |     15 |     12 |      8 |
| Student B |     12 |        |     16 |     10 |

where 10=(15*0.2+12*0.5+8*1)/(0.2+0.5+1) and 12=(16*0.5+10*1)/(0.5+1)

I just can't guess what has to be put in @3$2 as a column formula to
calculate those mean means… I perhaps have overlooked something simple.
Anyway, if anyone has a clue here, I will be pleased to hear it.

-- 
Nicolas Goaziou