Factorise:
`ar + br + at + bt`

Answer

To factorise the expression \( ar + br + at + bt \), we can follow these steps:

1. **Group the terms**: We can group the terms in pairs.
\[ (ar + br) + (at + bt) \]

2. **Factor out the common factors**: In each group, we can factor out the common factor.
- From the first group \( ar + br \), the common factor is \( r \).
- From the second group \( at + bt \), the common factor is \( t \).
\[ r(a + b) + t(a + b) \]

3. **Factor out the common binomial**: Now, we notice that \( (a + b) \) is common in both terms.
\[ (a + b)(r + t) \]

Thus, the factorised form of \( ar + br + at + bt \) is:
\[ (a + b)(r + t) \]