I have two tables: 
exam has fields : code,obt01.
hwsv has fields:code, perc
e.g

code         perc
1              100
2              89
3             70
.....
i want to update exam field  obt01 with  hwsv filed is perc.

like this.

if  perc is 100  replace exam field with 10


if  perc is 80  to 99  replace exam field with 9
if  perc is 70 to 89  replace exam field with 8
............