Interleaving is the practice of giving students activities that contain questions on a variety of different maths topics; essentially, mixed exercises. Interleaving has been shown to increase the retention of students’ learning and their ability to transfer it to other contexts.
The opposite of interleaving is blocked practice. In this case, students are taught a skill and then work on questions which all require the execution of that skill. Regardless of the context and numbers in the question, students already approach the question knowing the strategy they need to execute to solve it.
It is thought that the reason interleaving is effective is that not only do students have to execute the strategy to answer the question, but they also have to identify it in the first place. Working on interleaved activities gives students experience in selecting mathematical strategies, as well as executing them.
Prof Bob Bjork interview explaining the benefits of interleaving