UPSC Civil Services 2014 Prelims General Studies Paper II

Ina row 'A' is in the 11‌th position from the left and 'B' is in the 10‌th position from the right. If ‘A’ and ‘B’ interchange, then 'A' becomes 18‌th from the left. How many persons are there in the row other than 'A' and 'B' ?