When Diffy opens, there are four black numbers placed at the corners of a black square. Your first goal is to fill in the four blanks in the blue circles in the middle of each side of the black square.
Start with any side of the black square and cli...ck in the blank in the blue circle on that side and type in the difference of the black numbers on that side (the larger black number less the smaller one). So if the black numbers on the top side of the black square were 7 and 12, we would type 5 (= 12 – 7) in the blue circle between the 7 and the 12.
Note: There are more directions on the right side.[more]

This virtual manipulative uses plus-minus chips to demonstrate adding positive and negative values. Using this virtual manipulative you may: Solve a problem using chips or try a new problem.

With this manipulative, you can represent place value and exchanges in base 2, base 5, or base 10. Set the desired base with the up and down arrow buttons below the workspace. Chips in columns then represent units (1), bases (2, 5, or 10), and base-s...quared (4, 25, or 100), as indicated by the numbers at the top of each column.
Note that chips can be dragged within a column but not from one column to another.[more]

The goal of this puzzle is to put three numbers inside of each circle so that they add up to 0. Solve the puzzle by dragging the black numbers to the blank spaces. You cannot move the blue numbers.
When the three numbers inside any circle add up to ...0, the circle changes color.[more]

The goal of this puzzle is to put three numbers inside of each circle so that they add up to 21. Solve the puzzle by dragging the black numbers to the blank spaces. You cannot move the blue numbers.
When the three numbers inside any circle add up to... 21, the circle changes color.[more]

Circle 99 – A puzzle involving adding positive and negative integers to sum to ninety nine. Instructions are on the right side of this virtual manipulative.
The goal of this puzzle is to put three numbers inside of each circle so that they add up t...o 99.
Solve the puzzle by dragging the black numbers to the blank spaces. You cannot move the blue numbers. When the three numbers inside any circle add up to 99, the circle changes color.[more]

In this video, a teacher demonstrates constructing a factor tree for the number 40. Factor trees are used to find the prime factorization of composite numbers. The teacher explains the process step-by-step as the factor tree is created on chart pap...er. ( 2:08)[more]

Dueling Calculators – Visualize a dramatic simulation of the effect of propagating rounding errors. There are more instructions to the right of the manipulative.

This virtual manipulative poses problems requiring you to arrange numbers in positions in a diagram so that all numbers in a line add up to a given value.

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