Explanation:

- We have 15 taps in total, some are inlets, and some are outlets.

- Each inlet tap fills the tank in 30 hours. So, the rate of one inlet tap is 1/30 of the tank per hour.

- Each outlet tap empties the tank in 60 hours. So, the rate of one outlet tap is 1/60 of the tank per hour.

- We open all 15 taps, and it takes 4 hours to fill the tank completely.

- Let x be the number of inlet taps, then the number of outlet taps is 15 - x.

Use the equation based on their rates:

- x/30 - (15 - x)/60 = 1/4

Solve for x:

- Multiply through by 60: 2x - (15 - x) = 15

- Simplify: 3x - 15 = 15

- 3x = 30

- x = 10

- Answer: Option 3: 10