# Surf for More Math

## Lesson 7 - Subtracting 2-Digit Numbers

To encourage students to have fun on the Web while learning about Subtracting
2-Digit Numbers, here are some games and interactive activities they
can do on their own or in pairs.

**Goal **

Subtract 2-digit numbers with and without regrouping.

**Builds Upon **

Student Book pages 94-96

**Instructions for Use **

Subtracting
2-Digit Numbers provides students with strategies to mentally subtract
2-digit numbers.

To find the difference of the two numbers, click the 'Start Subtraction'
button. Input the answer in the text box next to the 'Check' button,
then click 'Check'. See if the answer is correct. Continue to find the
difference of the next two numbers.

The
Subtraction Machine lets students practice subtracting 1-digit and
2-digit numbers.

To use The Subtraction Machine, choose a skill
level, type the answer in the text field, and click the red button to
check it.

Base
Block Subtraction lets students practice subtraction with and without
regrouping.

To use Base Block Subtraction, drag a base block
into a column to the left. If students move a block one column to the
right, it breaks apart to show the exchange, for example, a 10-long
for ten units. To group pieces to exchange the other direction, click
and hold the mouse key down while dragging a rectangle to "lasso"
the pieces. The newly grouped object can then be dragged into the next
column.

To complete a preset subtraction problem, select base 10 and click
on the 'Next Problem' button. Move any pieces and drag them into the
next column to ungroup. Click, hold, and drag the mouse to move blue
blocks on top of red blocks of the same size to cancel each other out.
Blocks will disappear also, if a red block is placed on top of a blue
block of the same size. Start with the 1s column and work to the 1000s
column. The subtraction solution will appear on the left.

To subtract the blocks to create your own problem, click on the 'Create
Problem' button and then use the blue blocks at the top to enter blocks
above the dashed line. Click on the red blocks at the top to enter blocks
below the dashed line. As long as students has two numbers correctly
represented in the specified base, the subtraction problem will appear
on the left. To solve it, click on the 'Begin Problem' button and move
the blue blocks on top of the red blocks of the same size. Start with
the 1s column and work to the 1000s column. The subtraction solution
will appear on the left.