Question 157183
x=number of 6 volts batteries
y=number of 12 volts batteries

{{{6x + 12y = 84}}}
{{{x+y=10}}}
then
{{{x+2y=14}}}
{{{-x-y=-10}}}
then
{{{y=4}}}
{{{x=10-4=6}}}

Answer: there are  6 (6 Volts) batteries and 4 (12 Volts) batteries