Probability - Normal distribution curve?

Question

Can someon check my answer please -
mean median mode is 100gms standard deviation is 2 . Products less than 99g and above 100g are rejected. What is the probabilty of a product being rejected

1. 99-100/2 = -0.5 look up in table .6915 invert this(?) to give .3085

2. 105 - 100/2 = 2.5 look up in table .9938 invert this(?) to give .0068

3. add the two together to give .3147 probability of a product being rejected

many thanks

Answer 1

Was the question supposed to say reject product if less than 99g or more than 105g? If so your working is OK except that I don't like the term 'invert this' but I know what you mean.
I would simply say subtract from 1 to get the probability beyond this value.

Answer 2

Invert i guess u mean 1-phi(z)

1 is correct, as z<0 you do 1-

2 is correct aswell, seeing its above 105, you do 1-phi(105)

But i dont understand why you have done 105.

It says products less than 99g and above 100g so above 100g =1-0.5 - 0.5

So the probability of a product being rejected is 0.3085+0.5=0.8085

If you made a misprint in he question instead of 100 it is 105, you are correct at probability 0.3147.