Monthly maintenance cost for A is 950$ more than for B.

But every day A generates 100$ more income than B.

Then A needs to run at least 950/100 = 9.5 (rounded to 10) days to cover the difference in maintenance cost.

Bunuel wrote:

Machine A can produce 50 components a day while Machine B only 40. The monthly maintenance cost for Machine A is $1500 while the cost for Machine B is $550. If each component generates an income of $10, what is the least number of days per month that the plant has to operate to justify the usage of Machine A instead of Machine B?

A. 6

B. 7

C. 9

D. 10

E. 11