Use correct form of verb for :- (use is, are, was, were)
1. A pair of Trousers _____(be) displayed in the showroom.
2. Twenty thousand pounds _____(be) stolen in the robbery.

A pair of trousers (was) displayed in the showroom.
Twenty thousand pounds (was) stolen in the robbery.
1st sentence has a singular subject..i.e. a pair which will take singular verb.
2nd sentence has money associated in it which is never plural.