Mentat Wiki: Numbers Near Multiples Of Ten
It’s fairly easy to multiply two numbers that are close to the same multiple of 10.
The algorithm for doing it is called “Nikhilam Navatascaramam Dasata.” It is part of a system of algorithms and mnemonics to remember them, collectively known as “Vedic Math”, that was developed by Jagadguru Swami Bharati Krishna Tirthaji Maharaj in the early 20th century.