7.3. In 60 chandeliers (each chandelier has 4 lampshades), lampshades need to be replaced. Each electrician spends 5 minutes replacing one lampshade. A total of 48 electricians will be working. Two lampshades in a chandelier cannot be replaced simultaneously. What is the minimum time required to replace all the lampshades in all the chandeliers?
Answer: 25 minutes

Solution. Let's show how to proceed. First, 48 electricians replace one lampshade in 48 chandeliers, which takes 5 minutes, and 48 chandeliers have one lampshade replaced, while 12 have none replaced. Then, 12 electricians replace lampshades in the chandeliers that haven't been replaced yet, while the remaining 36 electricians replace the second lampshades in 36 chandeliers. This again takes 5 minutes, resulting in 36 chandeliers with two new lampshades and 24 with one. Now, 24 electricians replace the second lampshades, and 24 replace the third. Now, 24 chandeliers have three new lampshades and $36-$ have two. Now, 36 electricians replace 36 third lampshades and 12 electricians replace 12 fourth lampshades. Now, 48 chandeliers have three new lampshades and 12 have four. The final stage is 48 electricians replacing the last lampshades in 48 chandeliers. Thus, in 5 stages, i.e., 25 minutes, all lampshades in the chandeliers are replaced. Let's show that it cannot be done in less than 25 minutes. It is necessary to replace 60.4 = 240 lampshades. It takes 5 minutes for each lampshade, so in total, it is no less than $240 \cdot 5=1200$ minutes. But we have 48 electricians, so this can be done in $1200: 48=$ 25 minutes, but no less.