Let magazine be represented by m
Let ads be represented by a
Let the letter be represented by l
Let bills be represented by b
Three more ads than letters =>
a = 3 + l
Five more bills than ads
b = 5 + a = 5 + 3 + l= 8 + l
The same number of the magazine as letters mean that m = l
so if the sum of mail = 31
Therefore
3 + l + 8 + l + l + l = 31
4l = 31 - 11
4l = 20
l = 20/4 = 5
Since m = l
Then, m = 5
Thus the number of magazines they receive is 5