Answer:
6 cannot be used to write related number sentences.
Explanation:
The number 6 cannot be used to write related number sentences as it would not fit into the equation and make it true.
![7+8 =15](https://img.qammunity.org/2019/formulas/mathematics/middle-school/nhf312m4z688gyd6heh5blrr5q9dxe5l45.png)
![8+7 =15](https://img.qammunity.org/2019/formulas/mathematics/middle-school/r5ucm7dp6lkb1zcb88cm757emcnjrlunul.png)
![15-8 =7](https://img.qammunity.org/2019/formulas/mathematics/middle-school/hrz2c3fkla359rr98xqhd13z1765a0m2n2.png)
![15-7 =8](https://img.qammunity.org/2019/formulas/mathematics/middle-school/3kgcin2xhe9kejnp0gs3vcflhhry16oasn.png)
Since, nowhere in any of these equations can the 6 go in,
so that it make these number sentences true.