Josh F. answered • 02/06/21
Tutor
5.0
(204)
SAT and ACT test prep tutor with 10+ years of experience
Because you are adding rates, you are really adding the fractions 1/4 and 1/12. 1/4 + 1/12 = 1/3. So it will take them 3 hrs (the reciprocal of their joint rate).
It is sometimes useful to do this all with specific numbers, like this: Suppose the field has 12 rows. That means Molly does 3 rows / hr. Ashley does 1 row / hr. Together they do 4 rows /hr and 12 / 4 = 3 hrs. Same answer, different approach.
Josh F.
tutor
My pleasure — don’t forget that strategy of assigning a real number to job. It’s an effective strategy for many SAT math questions. Notice the choice of 12 here is convenient-“ — it’s the smallest number evenly divisible by 4 and 12 (ie the LCM).
Report
02/06/21
Molly M.
thank you so much!!02/06/21