Help!
posted by Frustrated mom! 4th grd MATH .
I am stumped with a problem we are supposed to help our 4th grader with ... Study the number of pairs: 3  8 ; 4  11; 5  14; 6  17; and the it has blanks down to 10  29; ... ok we get this part ... adding 2 more each set ... now the questions ... If 50 is the 1st number, what is the 2nd? ... If 200 is the first nimber, what is the 2nd? ...If 89 is the second number, what is the 1st? ... and finally, if a number n is the first number, what is the 2nd? ............ Any help with a specific equation would be most helpful and appreciated!!! ... thank you in advance! ...

3  8 difference = 5
4  11 difference = 7
5  14 difference = 9
etc.
In each case the difference is:
2 x (first number)  1 
Let x = first number
Let y = second number
x +(2x1) = y
3x1=y
so
x = (y+1)/3 
When they want y (the second number) when given x (the first number),
y = 3x 1