A piece of copper weighs 100 N in air, and 26 N in water. Find: a) Buoyancy; b) Density of copper

The weight in water of a piece of copper is equal to the difference between the force of gravity and the force of Archimedes.
P = Ft – Fa, P – weight in water (P = 26 N), Ft – weight in air (Ft = 100 N, Ft = m * g, m – mass of a piece, g – acceleration of gravity (g = 10 m / s ^ 2)), Fa is the Archimedes force (Fa = ρ * g * V, ρ is the density of water (ρ = 1000 kg / m ^ 3), V is the volume of a piece of copper).
Fa = Ft – P = 100 – 26 = 74 N.
V = Fa / (ρ * g) = 74 / (1000 * 10) = 0.0074 m ^ 3.
m = Ft / g = 100/10 = 10 kg.
ρm = m / V = 10 / 0.0074 = 1351 kg / m ^ 3.
Answer: The force of Archimedes is 74 N, the density of copper is 1351 kg / m ^ 3.