Last year Department Store X had a sales total for December that was 7 times the average (arithmetic mean) of the monthly sales totals for January through November. The sales total for December was what fraction of the sales total for the year?  Choose the correct answer
A) 1 / 4 B) 4 / 15 C) 1 / 3 D) 7 / 18 E) 4 / 5

Rationale: "let avg for 11 mos . = 10 therefore , dec = 70 year total = 11 * 10 + 70 = 180 answer = 70 / 180 = 7 / 18 = d"
Answer: D