What is the number halfway between 2.1 and 4.2?

The number halfway between 2.1 and 4.2 can be calculated by finding the average of the two numbers. Add 2.1 and 4.2 together to get 6.3, then divide by 2 to find the average, which is 3.15. Therefore, the number halfway between 2.1 and 4.2 is 3.15.