Necesito crear una aplicación para un control de las dosis de insulina con el siguiente esquema: control nivel glucosa en sangre cada día, al tercer día sumar los resultados y dividirlo entre 3, si el resultado es 0,80 o menor, restar 2 unidades de insulina. Si el resultado es entre 0,80 y 1,2 seguir con las mismas unidades de insulina. Si el resultado es mayor de 1,2 subir 2 unidades de insulina
Are you attempting to do something like the link (by the American Diabetes Association) ?
You will need to use the Math blocks and some If..then blocks and thoroughly test your results.
That algorithm link auto-expired.
Do you have a search query alternative?
Posted the link. However kasty should also search this google search for something similar. Sorry about the original link. I didn't realize it was short term. This is the original article.
It is very important for a person on insulin to follow his or her instructions exactly and your app inventor algorithm should reflect that. 
This is a great idea, but why reinvent the wheel?
https://www.google.com/search?q=insulin+app+play+store
Those would be better than risking health on beginner's code.