We want to find the combined rate. So in the example, if machines X and Y work together we know it will take less than 4 hours intuitively. The question is how long? So after 4 hours machine X completes 100% of the task. After 5 hours, machine Y completes 100% of the task. So to compare apples to apples, how much will machines X and Y complete in 1 hour? It is and of the task respectively. To find how much of the task they complete together in 1 hour, we add the two and get of the task completed. But this is only the amount they have finished in one hour. Then we set up a ratio and proportion to find how long it will take both of them to complete 100% of the task. You don't have to use 1 hour. We could have done the following:
How much will machines X and Y complete in 2 hours? It is and respectively. Then together in 2 hours they can complete of the task. Then we set up a ratio and proportion to find out how long it would take both of them to complete 100% of the task. The key idea is that we want to compare apples to apples (i.e. keep times constant to calculate rates).