He worked an 8 hour day 3 days.
Let x be the number of 8 hour days he works. Then 10-x is the number of 6 hour days he works.
8x + 6(10-x) = 66
Using the distributive property,
8x + 6*10 - 6*x = 66
8x + 60 - 6x = 66
Combining like terms,
2x + 60 = 66
Subtract 60 from both sides:
2x + 60 - 60 = 66 - 60
2x = 6
Divide both sides by 2:
2x/2 = 6/2
x = 3
There were 3 days he worked 8 hours.