Why can't schoolchildren use a formula to solve math problems that don't contain numerical values ?

Fifth and sixth graders were presented with 2 problems: (1) "Find the area of this triangle [base 8 cm, height 5 cm]," and (2) "How big is triangle B [an isosceles triangle] compared to triangle A [a right triangle]?" In figure accompanying the second question, the bases of the 2 triangles were equal, and triangle B was twice the height of triangle A. However, numerical values for the width of the base and the height were not provided. If the children were able to use the formula, "area of a triangle=base X height-f-2", properly, they should have been able to realize that triangle B had twice the area of triangle A. This is called the "relational operation of a formula". Of the 135 children, 123 (91%) gave the correct answer to Question 1, but only 62 (46%) answered Question 2 correctly. In other words, the children who could calculate area by using the formula could not necessarily operate the variable in the formula. Further analysis suggested that prerequisites for giving the correct answer to Question 2 were: (a) grasping the relative, rather than absolute, difference in the areas of the 2 triangles, and (b) understanding the difference between "the difference in the area of 2 figures" and "their ratio".