Calculate the standard emf for the following reaction:
2clo3- (aq)+10br-(aq)+12h+(aq) --> cl2(g)+5br2(l)+6h2o(l)

In order to solve this, we need to know the standard cell potentials of the half reaction from the given overall reaction.
The half reactions with their standard cell potentials are:
2ClO−3(aq) + 12H+(aq) + 10e- = Cl2(g) + 6H2O(l) 
E = +1.47 

Br(l) + 2e- = 2Br- 
E = +1.065

We solve for the standard emf by subtracting the standard emf of the oxidation from the reducation, so:
1.47 - 1.065 = 0.405 V

This is correct