is it possible in MySQL?

One of field of my database is amount.
How can I update (set mark="X") as many rows as sum of the amount give
me <=20? (in Id order DESC)


|id| amount |
|1 | 8 | update this
|2 | 7 | this
|3 | 3 | and this
|4 | 11 | but not this (>20!)