Answer
$(1-a)(1-b)
$
Work Step by Step
$\bf{\text{Solution Outline:}}$
Group the terms of the given expression, $
1-a+ab-b
,$ such that the factored form of the groupings will result to a factor common to the entire expression. Then, factor the $GCF$ in each group. Finally, factor the $GCF$ of the entire expression.
$\bf{\text{Solution Details:}}$
Grouping the first and second terms and the third and fourth terms, the given expression is equivalent to
\begin{array}{l}\require{cancel}
(1-a)+(ab-b)
.\end{array}
Factoring the $GCF$ in each group results to
\begin{array}{l}\require{cancel}
(1-a)+b(a-1)
\\\\=
(1-a)-b(1-a)
.\end{array}
Factoring the $GCF=
(1-a)
$ of the entire expression above results to
\begin{array}{l}\require{cancel}
(1-a)(1-b)
.\end{array}