8.4 out of 10
17 people- first test average = 2/10,
17+15= 32 people- total test average = 5/10
15 people retest average= ?
17x/17=2/10==> x=2/10
(17x+15y)/32=5/10==> 17*2/10+15y=16==> 15y=16-17/5, y=63/75=84/100 or 8.4/10
Retest average was 8.4 out of 10