I downloaded three apps I found on a list of apps you should have if you are teaching math. One seems to have disappeared and I've been unable to find it. The three I found are Ooops, Virtual Manipulatives, and iSpy-x.
The basic premise of Ooops is that you get several numbers and operations that are equal to something. You insert parenthesis to make the problem come out right. It has 5 levels and uses all four operations. Sometimes it uses one set of parenthesis, sometimes it uses two. When you think you have the answer, you hit the check button and it shows the work with the answer your problem has and tells you if it is right or wrong. I like this app in that it gives the students a better conceptual grasp of PEMDAS and how it can effect the results of the problem.
Virtual Manipulatives has actual manipulatives for fractions, decimals and percents. It uses the bars that start at 1 and then are subdivided for 1/2, 1/3, 1/4, etc. The 1/3 for fractions, decimals and percents has all the same color and you can move pieces so you can have students explore 1/2 = 1/4 + 1/4 or 50% = 10% + 10% + 10% + 10% + 10%. Virtual Manipulatives is great for the upper elementary or middle school grades. I could see using in high school with students who struggle with basic math or in a pre-algebra class.
Finally iSpy-x which is supposed to help students learn to solve simple problems such as 2 + x = 7. You can choose the one of three levels and on each level you can select the operation or used mixed operations. I downloaded the program 3 different times and each time it had a different bug. I could never get it to work on more than the easy level for addition.
I know that I am going to load Ooops on my iPads next year to use in the classroom. I like it and it can provide great differentiated instruction due to the 5 levels it offers.