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question A solution:
key: n=missing number /=divide by
mean=all numbers added together divided by the amounts of numbers
1+2+3+4+5+6=21 mean= 21/6. if there is one less number, then mean= (21-n)/5
replace n with the missing number
21-6=15/5. 15/5=3 *3 is less than 3.6 but fairly close*
21-5=16/5. 16/5=3.2 *3.2 is a bit closer to 3.6 so n<5
(when you put divisions into fractions and fractions into decimals, you realize that when dividing increasing numbers by 5, the outcome increases each time by 0.2 by knowing this, dividing the next numbers will be easy.)
21-4=17/5. 17/5=3.4 *n<4*
21-3=18/5. 18/5=3.6 this is the number we have been looking for. 3.6=(21-4)/5