Then you work with the Z-tables, using
where x is weight, is the mean and is the standard deviation.
For (a) you need to find the value of Z corresponding to a probability of 0.8,
since 80% of the bell-shaped curve lies below that.
For (b) you need the values of Z corresponding to 2.5% and 97.5%
since a symmetrical 95% of the graph lies in between (twice 47.5%).
For part (c) you must calculate Z corresponding to x=100kg, using the formula given above..
then read off the probability from the tables.
That will give you the probability of a weight being below 100kg.
Subtract from 1 and multiply by 100 to get the percentage.