Exchange rate on 11/25/2025:

$$\text{New Buying Rate} = \text{Original Buying Rate} \times \left(1 + \frac{\text{Devaluation Rate}}{100}\right)$$ $$\text{New Buying Rate} = 142.48 \times \left(1 + \frac{0.5}{100}\right)$$ $$\text{New Buying Rate} = 142.48 \times (1 + 0.005)$$ $$\text{New Buying Rate} = 142.48 \times 1.005$$ $$\text{New Buying Rate} = 143.1924 \text{ NRs./USD}$$

$$\text{New Selling Rate} = \text{Original Selling Rate} \times \left(1 + \frac{\text{Devaluation Rate}}{100}\right)$$ $$\text{New Selling Rate} = 143.08 \times \left(1 + \frac{0.5}{100}\right)$$ $$\text{New Selling Rate} = 143.08 \times (1 + 0.005)$$ $$\text{New Selling Rate} = 143.08 \times 1.005$$ $$\text{New Selling Rate} = 143.8054 \text{ NRs./USD}$$

Comparison:
After 0.5% devaluation, the new buying rate is NRs. 143.1924 per USD, and the new selling rate is NRs. 143.8054 per USD. Both the buying and selling rates have increased, meaning more Nepalese rupees are needed to buy 1 US dollar, and more Nepalese rupees are received when selling 1 US dollar. The selling rate (NRs. 143.8054) is still higher than the buying rate (NRs. 143.1924).
The final answer is $\boxed{\text{New Buying Rate: NRs. 143.1924, New Selling Rate: NRs. 143.8054. Both rates have increased due to devaluation.}}$.

We use the new buying rate calculated in part (c).

$$\text{Amount in NRs. (after devaluation)} = \text{Amount in USD} \times \text{New Buying Rate}$$ $$\text{Amount in NRs. (after devaluation)} = 3,000 \times 143.1924$$ $$\text{Amount in NRs. (after devaluation)} = 429,577.20$$

$$\text{Gain} = \text{Amount in NRs. (after devaluation)} - \text{Amount in NRs. (before devaluation)}$$ $$\text{Gain} = 429,577.20 - 427,440 \quad (\text{from part b})$$ $$\text{Gain} = 2,137.20$$

The tourist will gain NRs. 2,137.20.
The final answer is $\boxed{\text{NRs. 2,137.20}}$.