In Maths today one of our groups was given equations like the ones below. The group was able to answer the questions very quickly. The question was then asked - Why was it so easy?

Addend

+

Addend

=

Sum

   

1	16	+	?	=	20
2	46	+	?	=	50
3	32	+	?	=	40
4	72	+	?	=	80
5	61	+	?	=	70
6	81	+	?	=	90

In a group discussion Jeremy, Somatha and Timothy offered some could reasons. 

We all finally agreed that because the sum was always a group of TENs we could just look at the digit in the Ones place of the first addend and use our knowledge of what made a TEN to help identify the missing Addend.

Knowing the following FIVE basic Maths facts helped us solve 2-digit by 1-digit additions.

1 + 9 = 10

2 + 8 = 10

3 + 7 = 10

4 + 6 = 10

5 + 5 = 10

For Question 1 we just had to remember what goes with 6 to make a TEN.

For Question 3 we just had to remember what goes with 2 to make a TEN.

For Question 5 we just had to remember what goes with 1 to make a TEN.