Question 1193281
A. {{{a(x)=x*1.05}}} is the total, after taxes, on the purchase amount "x", not including the delivery fee in any way.

{{{b(x)=x+50}}}  is the total, before taxes, of the purchase amount and the delivery fee.

B. {{{b(a(x))=b(x*1.05)=(x*1.05)+50}}}

{{{a(b(x))=a(x+50)=(x+50)*1.05=(x*1.05)+(50*1.05)}}}

b(a(x)) is the lower cost because tax is not assessed on the delivery fees, which would add $2.50 to the total.

C. b(a(x)) would be used

Note: multiplying a number by 1.05 gives you the number plus 5% of the number.  Multiplying by just 0.05 would give you just the 5%