'one-half the product of eight and a number'
Replace the words "a number" by "N"
'one-half the product of eight and N'
Replace the word "one-half" by 
'
the product of eight and N'
Replace the words "the product of 8 and N" by "8N"
8N
Multiply
by the 8 and get 4 and put the N beside it, and get
4N
Edwin