How do you find the rule for a function? Then write the rule as a equation?

Question

My fifth grader came home with this problem:

Input a: 16 34 47 51 63

Output: r 82 64 51 47 35

Answer 1

For this, the simplest method is to add the input and output up:
16 + 82 = 98
34 + 64 = 98
47 + 51 = 98
51 + 47 = 98
63 + 35 = 98

Since they all add up to the same amount, the function is a straight addition/subtraction function. Thus, the function is: r = 98 - a.