1. Determine the lower bound of Rosie's money.
If she has £12 to the nearest pound, the actual amount she has, M, is in the range 11.50 ≤ M < 12.50.
The minimum amount of money she could have is £11.50.
2. Calculate the total cost of the 6 drinks.
Total cost = 6 × £1.89 = £11.34.
3. Compare the minimum amount of money with the total cost.
The minimum amount Rosie has is £11.50.
The cost of the drinks is £11.34.
Since £11.50 > £11.34, even in the worst-case scenario, she has enough money.
Therefore, Rosie definitely has enough money.
The phrase "to the nearest pound" implies we need to consider the limits of accuracy, or bounds.
Rosie has £12 to the nearest pound. This means the actual amount of money she has is between £11.50 (inclusive) and £12.50 (exclusive).
To be certain she has enough money, we must check if the minimum possible amount she has is enough to cover the cost.
The lower bound of her money is £11.50.
Next, we calculate the total cost of the drinks:
Cost = 6 drinks × £1.89/drink = £11.34.
Finally, we compare her minimum possible money with the cost:
£11.50 is greater than £11.34.
Since the least amount of money she could possibly have is more than the cost of the drinks, she definitely has enough money.
Expert