A poem wage for Gil = number of hours* wage per hour
= 1*10 = 10$
A poem wage for Holly = number of hours*wage per hour
= 2*10 = 20$
A poem wage for Ivan = number of hours * wage per hour
= 3*10 = 30$
And all 3 of them works for 12 hours per day.
Since Gil charges least for a poem, it is always betterr to take Gil for first 12 poems.
Let us find wage for 18 poems = (wage for 12 poems from Gil) + (wage for 6 poems from Holly) = 12*10 + 6*20 = 240$
Since both Gil and Holly worked for 12 hours to complete 18 poems.
Remaining 1 poem should be written by Ivan.
And that will cost 30$.
Hence It will cost 30$ more to commission 19 poems per day than 18 poems per day.