Example: Alejandro's father hired 15 workers who, working 40 days for 10 hours a day, built a swimming pool in his house with a capacity for 80,000 liters of water; If Alejandro hires 10 of these workers to work 6 hours a day and build another pool with a capacity of 40,000 liters of water, how many days will it take him to build it?
Workers: 15 // 10
Hours: 400 // x
Liters: 80 000 // 40 000
- (15/10)(2) = x/400
x = 12000 -> 1200/6 = 200 days
- (10/15)(2) = 400/x
x = 300 -> 300/6 = 50 days
The second equation is the correct one.
You have $15$ workers working for $400$ hours to create $80000$ liters of capacity. That means $6000$ worker-hours are necessary to create $80000$ liters of capacity.
Now you want to create $40000$ liters of capacity. That obviously will require $3000$ worker-hours. You have $10$ workers working $6$ hours per day, so you're getting $60$ worker-hours per day. That means that in $\frac{3000}{60}=50$ days, you'll have accumulated the $3000$ worker-hours you needed.
I find it easier to understand what's going on in the real world than to memorize formulae.