Marengo 12/10

Sir could you pls look at part a. d1= -0.0552 N(-d1)= 0.5+[0.0199+(0.0239-0.0199)/2]=0.5219 Dont you think if d1 was negative it should have been subtracted from 0.5 and other thing Is fimding value i know the .05 is 0.0199 and because the value of next five is not given so we take next no and subtract the first no and divide by two as this is almost in the middle ,am i right. What happens if it is .0572 do we still do same and divide by 2 aswell. Thanks