Answer:
Step-by-step explanation:

A experiment because he has a question to test and he can use observations and everything like that to test a question
Here is how to work the problem... basically you solve for one of the variables in one of the problems and then use substitution into the other. once you have both answers you substitute them both Into the last equation to solve.
Pretty sure it’s -12 but I could be reading the question wrong since it’s pretty late
" the sum " means add
16 + x = 20 <= ur equation
x = 20 - 16
x = 4 <== the number