H2SeO3 + 4Br^- + 4H^+ = Se + 2Br2 + 3H2O 

Hence the order coefficient (molar ratios) are ; 1:4:4::1:2:3 

Oxidation half equation: 2Br⁻ → Br₂ + 2e⁻

Reduction half equation: H₂SeO₃ + 4H⁺ + 4e⁻ → Se + 3H₂O

(Oxidation half equation)×2 + (Reduction half equation)×1, and cancel 4e⁻ on the both sides. The balanced overall equation is:

H₂SeO₃ + 4Br⁻ + 4H⁺ → Se + 2Br₂ + 3H₂O

The coefficients, in order from left to right, are _1, 4, 4, 1, 2, 3_.