Question 15154
To find the mean (or average) of a problem you add together the numbers then divide by however many numbers there are.  In this case you only have 3 of the 4 numbers you need.  Use x for the fourth.

{{{((70+60+40+x)/(4))= 72}}}

Then yoy want to solve for x.  Make the 72 a {{{72/1}}}.

{{{((70+60+40+x)/(4))= (72/1)}}} Then cross multiply.
{{{((170+x)/4)=(72/1)}}} 
{{{1(170+x)=(4)(72)}}}
{{{170+x=288}}}
{{{x=118}}}

If you want to check your answer you go back and plug in 118 for x. 

{{{(70+60+40+118)/4=72}}}

It works! So 118 is your answer.