Answer: To find what should be added to -2/8 to get 5/7, we can set up an equation as follows:
-2/8 + x = 5/7
where x is the number we need to add.
First, we can simplify both sides of the equation by finding a common denominator for -2/8 and 5/7. The least common multiple of 8 and 7 is 56, so we can rewrite the equation as:
-2/8 * 7/7 + x * 7/7 = 5/7 * 8/8
Simplifying the equation gives:
-14/56 + 7x/56 = 40/56
Next, we can simplify the left-hand side of the equation by combining like terms:
(7x - 14)/56 = 40/56
Now we can cross-multiply to solve for x:
(7x - 14) * 56 = 40 * 56
7x - 14 = 40
Adding 14 to both sides gives:
7x = 54
Dividing both sides by 7 gives:
x = 54/7
Therefore, to get from -2/8 to 5/7, we need to add 54/7, or approximately 7.71.