$32$ cells, each of $emf$ $3V$, are connected in series and kept in a box. Externally, the combination shows an $emf$ of $84\, V$. The number of cells reversed in the combination is
Medium
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Let $\mathrm{m}$ cells be in reverse
$(n-2 \,m) \varepsilon=84$
$(32-2 \,m) 3=84$
$4=2 \,m \Rightarrow m=2$
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