Given: AB ⊥ BO and NM ⊥ OM

In ∆ABO and ∆NMO,

∠OBA = ∠OMN

OB = OM …O is mid point of BM

∠BOA = ∠MON …vertically opposite angles

Thus by AAS property of congruence,

∆ABO ≅ ∆NMO

Hence, we know that, corresponding parts of the congruent triangles are equal

∴ AB = MN

Hence, we can calculate the width of the river by calculating MN