Question 880559
{{{x}}}= number of marbles John has.
The average number of marbles per person is the sum of all the marbles divided by the number of persons, so
{{{(x+55+78)/3=68}}} .
Multiplying both sides of the equal sign times 3 we get
{{{x+55+78=68*3}}} ---> {{{x+133=204}}} , and that is the total number of marbles.
Subtracting 133 from both sides of the equal sign we get
{{{x=204-133}}} ---> {{{x=highlight(71)}}} and that is how many marbles John has.