How much would it cost to build a driveway 12 feet wide by 150 feet long and 6 inches thick if the?

yd = yard
ft = foot
in = inch

1 yd = 3 ft
1 ft = 12 in

The area a in ft² is then

12 ft * 150 ft = 1800 ft²

And in yd²

4 yd * 50 yd = 200 yd²

finishing costs are $2.00 per square foot:

$2.00 * 1800 ft² = $3,600.00
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The concrete layer shall be 6 in thick and there are (3 * 12 in = 36 in) in a yard.

1 yd³ = 1 yd² * 36 in, and

36 in / 6 in = 6 layers of concrete, 1 yd² each. So, 1 yd³ will suffice for 6 yd² of the driveway. There are 200 yd² :

200 / 6 = 100 / 3 ≈ 33.33 yd³ of concrete will be needed.
Concrete cost is $135 per cubic yard:

$135 * 33.33yd³ = $4,499.55

We round generously up to: $4,500

and add the finishing cost:

$4,500.00 +
$3,600.00
----------------
$8,100.00

total cost for your new driveway.