How can you maintain the level of questions if they are different?

Let us explain that with a maths problem (since that’s something we are good at! )
Problem 1: Rob can do a job in 2 days and Charlie can complete the same job in 5 days. How long will they take to complete the job working together?
Problem 2) Peter can do a job in 3 days and Mark can complete the same job in 4 days. How long will they take to complete the job working together?
As you can notice, both have the same logic, similar working and hence qualify as 2 questions at the same difficulty level from the same topic.