Q85. Let $A = \{ 2,3,6,7 \}$ and $B = \{ 4,5,6,8 \}$. Let $R$ be a relation defined on $A \times B$ by ( $\left. a _ { 1 } , b _ { 1 } \right) R \left( a _ { 2 } , b _ { 2 } \right)$ if and only if $a _ { 1 } + a _ { 2 } = b _ { 1 } + b _ { 2 }$. Then the number of elements in $R$ is $\_\_\_\_$\\