Finding standard deviations

[Ematuro]

I do not get how to do these find the standard deviation questions. Got stuck on this one:

The weight of a packet of biscuits is known to be normally distributed with a mean of
1 kg. If a packet is more than 0.05 kg underweight, it is unacceptable. If it is found that
3% of packets are unacceptable, then the standard deviation of the weight is:

Can someone please help? And when does one need to use the z formula I'm really confused.
Thanks in advance.

[luken93]

Pr(X < 0.95) = 0.03

Using invnorm for standard normal distribution, you'll find that the value of z such that Pr(Z < z) = 0.03 is −1.881

Therefore, using z = (x - u)/q

-1.881 = (0.95 - 1)/q
q=0.0266