It is much easier sometimes to write the numbers in column form. That is:
#color(white)(2)1. color(white)(2)2color(white)(2)4#
#color(white)(2)5. color(white)(2)3color(white)(2) 6#
#color(white)(2)ul(2. color(white)(2)4color(white)(2)2) larr" add up the columns"#
#color(white)(2)ul(color(white)(2" "))#
#color(blue)("'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~")#
#color(blue)("Step 1: add up the rightmost column")#
#4+6+2=12 -> color(red)(1)color(blue)(2)#
The #color(blue)(2)# goes in the right most column and the #color(magenta)(1)# is included in the adding up of the next left column
#color(green)(color(white)(2)1. color(white)(2)2color(white)(2)4)#
#color(green)(color(white)(2)5. color(white)(2)3color(white)(2) 6)#
#color(green)(color(white)(2)ul(2. color(white)(2)4color(white)(2)2)) larr" add up the columns"#
#color(white)(2)ul(color(white)(2" "color(blue)(2)))#
#" "color(red)(1)#
#color(blue)("'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~")#
#color(blue)("Step 2: add up the next left column")#
#2+3+4+color(red)(1)" " larr" don't forget the carried 1"#
#2+3+4+1=10->color(red)(1)color(blue)(0)#
#color(green)(color(white)(2)1. color(white)(2)2color(white)(2)4)#
#color(green)(color(white)(2)5. color(white)(2)3color(white)(2) 6)#
#color(green)(color(white)(2)ul(2. color(white)(2)4color(white)(2)2)) larr" add up the columns"#
#color(white)(2)ul(" "color(blue)(0)color(white)(.)color(green)(2))#
#color(white)(.)color(red)(1)color(white)(....)color(green)(1)#
#color(blue)("'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~")#
#color(blue)("Step 3: add up the last column")#
#1+5+2color(red)(+1)" " larr" don't forget the carried 1"#
#1+5+2+1= color(blue)(9)#
#color(green)(color(white)(2)1. color(white)(2)2color(white)(2)4)#
#color(green)(color(white)(2)5. color(white)(2)3color(white)(2) 6)#
#color(green)(color(white)(2)ul(2. color(white)(2)4color(white)(2)2)) larr" add up the columns"#
#ul(color(white)(.)color(blue)(9.)color(white)(.)color(green)(0)color(white)(.)color(green)(2)) larr" don't forget the decimal point"#
#color(white)(.)color(red)(1)color(white)(...)color(green)(1)#