In each blank below a single digit is inserted such that the following six three-digit numbers in this order, form an arithmetic sequence:
125, 129, 1_ _, 1_ _, 1_ _, 1_ _
What are the missing digit?
The common 'd' is 4 125 + 4 = 129 +4 = 133 +4 = 137 etc
125, 129, 133, 137, 141, 145
