Question 886770
rate * time = quantity.
quantity = 1



time for crispin to clean is 12 minutes.
formula becomes:
r * 12 = 1
rate for crispin to clean is 1/12 of the job per minute.


time for tonio to clean is 8 minutes.
formula becomes:
r * 8 = 1
rate for tonio to clean is 1/8 of the job per minute.


working together, crispin and tonio clean the rest of the job in 4 minutes.
their combined rate is (1/8 + 1/12) which is equal to (5/24) of the job per minute.


in 4 minutes they have cleaned 4 * 5/24 = 20/24 of the job.


this means that crispin must have already cleaned 4/24 of the job before tonio arrives.


4/24 can be simplified to 1/6 of the job.