Question 451512
First clerk finishes {{{1/45}}} of the shelf in one minute, while the second clerk finishes {{{1/25}}} of the shelf in a minute.

Together they would finish {{{1/45 + 1/25}}} of the shelf in a minute. 

{{{1/45 + 1/25}}} = {{{10/450 + 18/450}}} = {{{28/450}}}

Now we have a proportion that they would finish {{{28/450}}} of the shelf in one minute and we have to find how long it would take them to finish a whole shelf:

28/450 : 1 = 1 : x

From here

{{{x = 450/28}}} = 16.07 minutes