Let's use the variable "x" to represent the number of 3-inch bolts that James bought.
We know that he bought a total of 16 bolts, so the number of 4-inch bolts he bought would be 16 - x.
The cost of the 3-inch bolts is 36 cents each, so the total cost of the 3-inch bolts would be 0.36x dollars.
Similarly, the cost of the 4-inch bolts is 42 cents each, so the total cost of the 4-inch bolts would be 0.42(16 - x) dollars.
We know that the total cost of all the bolts was $6.00, so we can set up the following equation:
0.36x + 0.42(16 - x) = 6.00
Simplifying and solving for x, we get:
0.36x + 6.72 - 0.42x = 6.00
-0.06x = -0.72
x = 12
Therefore, James bought 12 of the 3-inch bolts.