Two identical cells of emf 1.5 V each joined in parallel, supply energy to an external circuit consisting of two resistances of 7Ω each joined in parallel. A very high resistance voltmeter reads the terminal voltage of cells to be 1.4 V. Calculate the internal resistance of each cell.

We know that internal resistance of a cell is given by
r=R(E/V-1)
Where
R=total external resistance ,E=total emf of cell ,V=terminal voltage ,
given
E=1.5V,V=1.4V
now as two given resistances are in parallel therefore total external resistance ,
R=(7×7)/(7+7) = 49/14 Ω
therefore
r=(49/14)((1.5/1.4)-1) = 0.25 Ω
r=total internal resistance of the cells,
now the two cells are in parallel therefore internal resistance of each cell ,
r=r'×r'/(r'+r')
or
0.25=r'²/2r'
or
r'=0.50 Ω