Use compatible numbers to estimate the quotient. 412 divided by 5
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First set up the equation to find answer: y=7-x. Then add it to the variable. 3x-1(7-x)=1. So that makes it 3x+-7+-1=1. Add like terms -7+-1=-8, so 3x+-8=1. Now to find x just switch the 8 to the other side. So you end up with 3x=9 since when you take away a number from one side you need to use the opposite value so adding 8 to -8 will cancel it out, then add it to the other, then just divide. 9/3=3, from here just substitute to find y. y=7+3. y=10. The answer is x=2 and y=5. I hope I helped!
First of all, note that all integers are either 0,1, or 2 modulo 3 (if you're not familiar with this terminology, it means that every integer is either a multiple of 3, or it is 1 or 2 away from a multiple of 3).
So, we can think of our numbers as
In order to make sure that the sum of any three adjacent numbers is divisible by 3, we have to make sure that any group of 3 three adjacent numbers contains a 0, a 1 and a 2. This is possible only if we arrange our 9 numbers in 3 groups of 3 numbers containing 0,1 and 2 exactly once, repeating always the same pattern.
For example, we could arrange our numbers following the pattern
or
We have possible patterns. Suppose for example that we choose the pattern
One possible way of following this pattern would be the arrangement
In fact, we substituted every '0' with a multiple of 3 (3, 6 or 9), every '1' with a number 1 away from a multiple of 3 (1, 4 or 7) and every '2' with a number 2 away from a multiple of 3 (2, 5 or 8).
This means that, once we fix a patter, we have 3 choices for the first 3 slots, 2 choices for the next 3 slots, and the final slot will be fixed. So, we have
possible ways of following a fixed pattern. Since the number of patterns was 6, we have
possible arrangements.