By the end of Reception, it is expected that children can recall the number bonds to 5. 

 0 + 5 = 5

1 + 4 = 5

2 + 3 = 5

3 + 2 = 5

4 + 1 = 5

5 + 0 = 5

Having this knowledge stored in the long term memory, allows children to work through addition and subtraction with more fluidity, and confidence with their Maths when learning other concepts.