Answer:
Associative propertry of addition.
Explanation:
The associative property of addition states that:
![a+(b+c) = (a+b)+c](https://img.qammunity.org/2020/formulas/mathematics/high-school/kcmoa7icad77togl0lw44vorz134aax0h1.png)
It states that we can add numbers regardless of the groups. The sum does not change irrespective of how the addends are grouped.
We are given the expression:
![r + (h + t)](https://img.qammunity.org/2020/formulas/mathematics/college/xrlvy13330ypahdulhhpunob6be8zrhycb.png)
It can be written as:
![(r+h) + t](https://img.qammunity.org/2020/formulas/mathematics/college/8fbsqp75x7i8fhnx0pcaxrs2ng2u5cudz7.png)
It can also be written as:
![r + (h+t) = (r+t) + h = (h+t)+r = h + (t+r)](https://img.qammunity.org/2020/formulas/mathematics/college/xo0xwfarjiswsl2fpjyabbfqde3mopdvhy.png)
Whatever maybe the arrangement of r, h and t, their sum will always be equal. We can regroup them in many ways but the sum remains the same.