19 A brand of orange juice is available in shop $A$ and shop $B$ at an original price of $\$ 2.00$ per bottle. Shop $A$ provides the "buy 4 get 1 free" promotion and shop $B$ provides $15 \%$ discount if one buys 4 bottles or more. Find the minimum cost (in cents) if one wants to buy 13 bottles of the orange juice.
19. Ans: 2160
In order to get the best price, the number of bottles bought in shop $A$ should be a multiple of 4 . We see that in the 3 cases:
(i) 0 from shop $A$ and 13 from shop $B: 2600 \times 85 \%=2210$,
(ii) 4 from shop $A$ and 8 from shop $B: \quad 800+1600 \times 85 \%=2160$,
(iii) 8 from shop $A$ and 3 from shop $B: 1600+600=2200$,
the lowest price is 2160 .