Question 437868
Since the batteries are connected in series the total voltage is equal to the sum of voltages connected in series:

{{{U=U[1]+U[2]+U[3]}}}

Let {{{U[3]=x}}} volt the voltage of third battery then the voltage  of the first 

is {{{U[1]=(2/3)*x}}} and the voltage of the second is {{{U[2]=(1/3)*x}}} 

Substitute and have:{{{54=(2/3)x+(1/3)x+x}}}, solve this equation:

{{{54=2x}}} => {{{x=27}}}

Answer: The third battery is 27 v, the second is {{{(1/3)*27=9v}}} and

the first is {{{(2/3)*27=18v}}}