The speed of a longitudinal wave in a solid is given by v=ρY where Y is Young's modulus and ρ is density.
Initial condition: v0=ρY=400 m/s
After changes: ρ′=1.005ρ (0.5% increase) and Y′=1.01Y (1% increase)
New speed: v′=ρ′Y′=1.005ρ1.01Y=1.0051.01⋅ρY
v0v′=1.0051.01=1.00498...≈1.00249
v′=400×1.00249≈401 m/s