Ambiguous Dates

In this country a date such as July 4,1971, is often written 7/4/71, but in other countries the month is given second and the same date is written 4/7/71. If you do not know which system is being used, how many dates in a year are ambiguous in this two-slash notation?

Answer:

Each month has 11 ambiguous dates (a date such as 8/8/71 is not ambiguous), making 132 in all.