Thủ thuật toán học:
[✔] x + y
[✔] x - y
[✔] x * y
[✔] x / y
[✔] x * 11
[✔] x * x
x = 15, 25, 35, 45, 55, 65 ..
[✔] x * 5
[✔] x * 9
[✔] x * 4
[✔] x / 5
[✔] 1000 - x
[✔] x * y
y = 2, 4, 8
[✔] 2 ^ x
[✔] x + y
y = 91 ~ 109, 191 ~ 209, 291 ~ 309 ..
[✔] x - y
y = 91 ~ 109, 191 ~ 209, 291 ~ 309 ..
[✔] x * y
x,y = 10 ~ 19
[✔] x * x
x = 11 ~ 99
[✔] x1x2 * y1y2
x1 = y1 = 1 ~ 9, x2 + y2 = 10
23 * 27, 59 * 51, 37 * 33, 68 * 62 ..
[✔] x * x
x = 50 ~ 59
[✔] x * x
x = 40 ~ 49
[✔] x * y
x,y = 11, 21, 31, 41, 51, 61, 71, 81, 91
[✔] x% -> y
[✔] x * x
x = 100 ~ 109

The app uses the following APIs and libraries:
* MPAndroidChart - created by Philipp Jahoda
* ColorPIcker - created by Piotr Adamus