ratio and propoertion or logic
ratio
cost:amount=16.5/50
16.5/50=x/45
times both sides by 50*45
16.5*45=50x
742.5=50x
divide both sides by 50
14.85=x
the answer is $14.85 for 45lb
or, math
16.5 for 50
1.65 for 5
50-5=45
16.5-1.65=14.85
answer is $14.85 for 45lb