Let x be the number of adults. Then the number of children is 2x and the number of students is (287-x-2x) = 287 - 3x. Now you can write the equation for total money 5*(2x) + 7*(287-3x) + 12*x = 2072. Simplify and solve for x 10x + 7*287 - 21x + 12x = 2072 x = 2072 - 7*287 = 63. ANSWER. 63 adults, 2*63 = 126 children and 287-63-126 = 98 students.
This problem is to be solved using one equation and one unknown.