The mass fraction of nitric acid in the solution obtained after adding 20 g of water to 320 g of its 2.5% solution is

w = m (in-va) / m (solution)
m (in-va) = 320 * 0.025 = 8g
The new mass of the solution is 320 + 20 = 340g
w8 / 340 = 0.0235 or 2.35%
The answer was asked to be accurate to the tenth, which means 2.4%

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