你的这个句子的谓语动词既可以用复数were,也可以用单数was。当“a pack of+复数名词”作主语侧重整体时，谓语动词用单数；若侧重个体，谓语动词用复数。如：

A pack of cigarettes lasts him two days.  一包香烟够他抽两天的。《简明英汉词典》

This pack of clothes is mine.  这包衣服是我的。《简明英汉词典》

A pack of reporters were waiting outside. 一群记者正在外面等着。《朗文当代英语词典》

A pack of journalists was waiting outside the White House. 一群记者正在白宫外面等着。《剑桥高级学习词典》

▲ 请看以下高中教材中的两个例句：

高中英语第一册“A Little Hero”（试用版）

A group of Italian soldiers was advancing slowly towards the position of the enemy. （侧重整体）

高中英语第一册“A Little Hero”（1985版）

A group of Italian soldiers led by a captain were advancing slowly toward the position of the enemy.（侧重个体）