When One Number Is Near a Hundred

There are at least two useful methods for solving such a problem. For example, in the case of 78 + 99: either 78 + 100 = 178, but this is one too many, so the answer is 177, or transfer one from the 78 to the 99.So 78 + 99 is the same as 77 + 100 = 177.

Using Materials

Problem: “Work out $99 + $78.”
Record 99 + 78 on the board or modelling book. Draw an empty number line on the  board. Show on the number line why 100 + 78 is one too many. Record the answer on the board or modelling book.
Examples: Draw empty number lines and work out: 56 + 99,  67 + 98,  97 + 63, 38 + 298,  123 – 99,  456 – 98 ...
Problem: “Work out $98 + $38.”
Record 98 + 38 on the board or modelling book. The students model $98 and $38 with play money. Discuss what transferring $2 from the $38 to the $98 does and how this shows that the answer is $136. Record the answer on the board or modelling book.
Examples: Using play money, work out: 151 – 99,  62 + 98,  97 + 73,  338 – 98,  103 + 197,  295 + 456 ...
 

Using Imaging

Problem: “Work out $175 – $99.”
Encourage the students to image this in two ways, namely, by number line and using play money, and to choose the method they like better.
Shielding: Examples: Word stories and recording for: 141 – 98,  55 + 98,  97 + 86,   18 + 298,  603 – 198,  299 + 456 ...

Using Number Properties

Examples: Word stories and recording for: 834 – 99,  456 – 98,  297 + 198, 818 + 698,  1 000 – 797,  1 200 – 998 ...