A vending machine sells three items: Code Item Price (pence) A Drink 120 B Crisps 85 C Chocolate 100 The machine accepts coins of 10p, 20p, 50p and 100p. Th...

(a) The vending machine algorithm collects coins, validates them, accepts an item selection, and calculates change. [8]

DECLARE TotalInserted : INTEGER
DECLARE Coin : INTEGER
DECLARE Selection : CHAR
DECLARE Price : INTEGER
DECLARE Change : INTEGER
DECLARE ValidCoin : BOOLEAN

TotalInserted ← 0

REPEAT
    REPEAT
        OUTPUT "Insert coin (10, 20, 50, 100) or 0 to select item: "
        INPUT Coin
        ValidCoin ← (Coin = 10 OR Coin = 20 OR Coin = 50 OR Coin = 100 OR Coin = 0)
        IF ValidCoin = FALSE THEN
            OUTPUT "Invalid coin"
        ENDIF
    UNTIL ValidCoin = TRUE
    IF Coin <> 0 THEN
        TotalInserted ← TotalInserted + Coin
        OUTPUT "Total: ", TotalInserted, "p"
    ENDIF
UNTIL Coin = 0

REPEAT
    OUTPUT "Select item (A, B, C): "
    INPUT Selection
UNTIL Selection = 'A' OR Selection = 'B' OR Selection = 'C'

CASE OF Selection
    'A': Price ← 120
    'B': Price ← 85
    'C': Price ← 100
ENDCASE

IF TotalInserted < Price THEN
    OUTPUT "Insufficient funds. You inserted ", TotalInserted, "p. Price is ", Price, "p"
ELSE
    Change ← TotalInserted - Price
    OUTPUT "Dispensing item"
    IF Change > 0 THEN
        OUTPUT "Change: ", Change, "p"
    ENDIF
ENDIF

The inner loop validates each coin before accepting it [1]. The outer loop accumulates the total until the user enters 0 [1]. The CASE statement maps the selection to its price [1]. Change is computed as the difference between the amount inserted and the price [1].

(b) Trace for coins 50p, 50p, 50p and selection "B": [3]

Output: "Dispensing item" followed by "Change: 65p" [1].

(c) The change is 65p [1].