click the green flag to start click on the numbers 0-9 to generate the first # then chose your calc function (+, - X, or /), click the 2nd number and click the equal (=) sign to find our the result. note that 'null' appears before you enter the first number and before you enter the second ... I made this selection to ease the programming. C clears the calculator ... click C before your next calculation!
check my original calculator here : https://scratch.mit.edu/projects/12590430