How a "bonus point" is won in an ODI :-
If the winning team can keep their run rate 1.25 times more than the team they beat - they win the bonus point from that match. For example, If Australia bats first and make 250 runs, Indian team needs to win the match at a run rate of (250/50)* 1.25. Which equals to 6.25 runs per over. Which in turn means that India must win the match in 40.1 overs (251/6.25) or of course - less than that.
In case India bats first and needs to win the bonus point after scoring 250 from 50 overs, they need to restrict Australia from scoring more than 187 runs. This is calculated as India's batting run rate was 5. To have a run rate 1.25 time better than Australia, India must not let Australia score more than 3.75 (5-1.25) as a run rate. Thus a total of no more than 187 from 50 Overs for Australia while batting second.
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Chase 189 in 35 overs