Find the mass fraction of a substance if 20 grams of water were added to 260 grams of an 80% solution.

Given:
m1 solution (in-va) = 260 g
ω1 (in-va) = 80%
m (water) = 20 g

To find:
ω2 (in-va) -?

1) m1 (in-va) = ω1 * m1 solution / 100% = 80% * 260/100% = 208 g;
2) m2 (in-va) = m1 (in-va) = 208 g;
3) m2 solution (in-va) = m1 solution (in-va) + m (water) = 260 + 20 = 280 g;
4) ω2 (in-va) = m2 * 100% / m2 solution = 208 * 100% / 280 = 74%.

Answer: The mass fraction of the island is 74%.