I first add the three numbers [it is three because I don't include the blank number] and write a 'guess 'number in the first blank spot. My guess number is not bigger than the biggest number and also not smaller than the smallest number.
For the second blank spot: when I add my guessed number [from the first blank] to the other three numbers, the answer should be in the table of 4's. If it is not, I have to re-guess the first number. Now do the calculations and ALWAYS remember the numbers that you write are NEVER going to be bigger than the biggest number OR smaller then the smallest. Once you have calculated what did you find out? Did you find out you were correct or were you incorrect and just need to try again with other numbers?