Let \( a_{1}, a_{2}, \ldots \ldots . ., a_{15} \) be in an arithmetic progression. \( a_{1}+a_{2}+\ldots \ldots .+a_{10}=A \) and \( a_{6}+a_{7}+\ldots \ldots . .+a_{15}=B \) . If \( B-A=200 \) and \( B+A=860 \) , then the value of \( a_{15} \) is