A key concept is to understand that addition can be done in any order. So 6 + 3 is the same as 3 + 6.
Recognising pairs of numbers that total 10 makes mental addition much easier.
Knowing that addition can be done in any order helps enormously when mentally adding 3 or more numbers.
Adding 10 and then adding 1 is the easiest way of adding 11 to a 2-digit number.
Adding 19 is a little bit harder, but by adding 20 and subtracting 1 it becomes much easier.
Add 21 in two steps: add 20 and then add 1, or you could add 1 and then add 20, which is easier will depend on the numbers.
Several ways this can be tackled. Try adding the two numbers and then counting on in tens to 100. Use the number line to help.
Find two numbers to make a total of 100. Lots of different correct answers are possible for this activity. Subtract the first number from 100 and then choose two numbers that make this answer.
Find two more numbers that will make the total 20. use the number line to help.
Find the missing numbers can be worked out in several ways. One good way is to add the first two numbers and then count on to the total given. Or you could start with the total and subtract each number in turn.
Use all the strategies learned to make finding the total for 3 items of shopping.