Question 163277
Let the fourth number be x.
Then the average of the four number can be expressed as:
{{{(216 + 159.8 + 301.25 + x)/4}}}
As the average is 638.4, we have
{{{(216 + 159.8 + 301.25 + x)/4=638.4}}}
Solving the equation for x, we obtain
{{{(677.05 + x)/4=638.4}}}
{{{677.05 + x=2553.6}}}   (multiply both sides by 4)
{{{x=1876.55}}}