The efficiency of a cell when connected to a resistance `R` is `60 %`. What will be its efficiency if the external resistance is increased by six times?

Explanation

Effiency `= eta = ( output power)/( Input power )`
`eta = (i^(2) R)/(epsilon i) where I = (epsilon)/( R + r )`
`eta = ( R)/( R + r ) or 0.6 = ( R ) /( R + r ) or 2 R = 3 r `
Therefore, new efficiency
`eta_(1) = ( 6 R) /( 6 R + r ) = 0.9 = 90%`