Solve this simple equation (help pls)

[Whateverist]

Not exactly sure what I did tbh Lol
Hold up I'll post the problem see if you guys get 15 as well

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So this is the original problem I did some bla bla bla calculations and the answer I got(you guys got 😛) was 15

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See if you guys get 15 as well

Worker A completes the job at a rate of 1/20 of the job/day.  Worker B completes 1/30 of the job each day.  So you want to know how many (1/20 + 1/30) it will take to complete one job, assuming the two can work compatibly with no added or lost efficiency.  If you let x = the number of days needed to complete the job working together this way, then a better equation for the problem would be:

(1/20 + 1/30)x = 1

Since 1/20 + 1/30 = 50/600 which reduces to 1/12, you'd get 1/12 of the job done per day this way.  So your original answer of 12 days was correct.  But your original equation wouldn't get you there.