Represent the number by let say
"twice a number decreased by seventeen"
There are several wordings for the same expression
"Twice a number decreased by seventeen"
"Seventeen less than twice a number"
"Double the number diminished by seventeen"
God bless....I hope the explanation is useful.
Breaking down the question into its component parts.
Let the unknown number be represented by
Twice a number
decrease by 17
Written as per convention this is:
As you can see from the above. The word decrease is mathematically expresses as 'subtract'
To subtract is the same as 'remove'. If you remove you have made the whole less. So the whole has been made to decrease.