its cement:fine agg:coarse agg
if we take 1 bag of cement=50 kg
then
cement=50 kg
fine agg=50*4=200kg
coarse agg=50*8=400kg
water cement ratio=0.4
w/c=0.4*50=20 liters of water

1 4 8 means cement sand (fine aggregate) metal 20 mm (coarse
aggregate)respectively.
The ratio is based on volume and not in weight.
one bag of cement is 1.25 cubic foot
Therefore multiply the fine and coarse aggregate with 1.25.
Thus you have to use 1.25 cft of cement (one bag) and 5 cft
of sand and 10 cft of coarse aggregate.
As for sand is concerned bulkage allowance is to considered
according to which the quantity may slightly vary

Nominal mix propertion of M10 PCC grade is 1:3:6 & Nominal
mix propertion of M .75 Is pcc grade for 1:4:8
Normally Mix design consider for above M20 grade as for Is
code 456

For M-10 grade nominal mix design is 1:3:6
For M-7.5 grade nominal mix design is 1:4:8

As per IS 456:2000, Nominal mix concrete may be used for concrete grades M-20 or lower. Design mix can be arrived with several trial mix design as per guideline proposed in IS 10262:2009. Example my trail design mix for M 10 is 1:3.5:6.10
Hope this will be useful for you...