hmm...

Sam does 1/4 of a job per day (job per day is like miles per hour, if you want to think of it that way)

Lisa has a speed of 1/6 of a job per day

Tom has a speed of 1/2 of a job per day

So, they complete the job amount of days later...

Let's solve for n, first we multiply both sides by 12 to get:

Then add:

Then divide:

So they can finish the job in