Say, there is a no ball in the very first ball of an innings. Total score becomes 1, over 0.0 What should be the current run rate at this moment ?
Run rate is defined as runs / (legal deliveries * 6) - i.e. runs per over. As the number of legal deliveries is zero, the current run rate is undefined. This is just the same as (e.g.) how a batsman's average is undefined if they have played only one innings and were not out - but frankly, it doesn't matter because everything fixes itself once a legal delivery occurs.
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The team could end with run rate of infinity if a legal ball is not bowled afterwards. For example, in the 4th innings of a Test match where the (small) target is achieved off the no-ball(s), or when play is interrupted immediately following the no-ball and never resumes. But even so, it doesn't really matter, as scoring rates for teams are of interest when calculated over a number of innings. (Example: "The Aussies under Steve Waugh often scored at over 4 runs per over in Tests, setting up more winning positions than other teams.") Nobody really cares about scoring rate in a single innings. – Masked Man Jan 21 '17 at 17:50