Question 41627
First write the variation equation: 
{{{C = k/n}}} where: C = cost, n = number of people, and k is the constant of variation.  To find the value of k, just substitute the given values of C = $31 and n = 50.

{{{31 = k/50}}} Multiply both sides by 50.
{{{k = 1550}}}

The equation of variation for the cost is:
{{{C = 1550/n}}}

Now you can find the cost for 81 people by substituting n=81.

{{{C = 1550/81}}}
{{{C = 19.14}}}

It will cost $19.14 per person if 81 people go on the trip.