According to the question, If B throws 1, then A is allowed to throw only 1 , if B throws 2, then A can throw 1 and 2 and soon. Hence, required probability. 61​⋅61​+61​⋅62​+61​⋅63​+61​⋅64​⋅61​⋅65​+61​⋅66​=6×61​(1+2+3+4+5+6)=6×6×26×7​=127​