F.4 math---inequalities3

用戶36732

2009-12-27 1:16 am

(1)

Mr. Lee drove from town A to town B at 65km/h. Half an hour later, Mr. Ho drove along the same route from town A to town B at 80km/h. At least how long would it take for Mr. Ho to be not less than 65km ahead of Mr. Lee?

(2)

Ivan and Tony have $x and $(x+50) respectively. It is known that half of the amount with Tony is at least $10 more than the amount with Ivan, and the sum of the amounts with them is greater than $60.

(a) Find the range of the amount with Ivan.

(b) Find the range of the amount with Tony.

回答 (1)

用戶9112

2009-12-27 2:02 am

✔ 最佳答案

(1) Let t be the number of hours Mr Ho started off from town A
The distance of Mr Lee with respect to the departure time of Mr Ho is (t + 0.5)*65 km = 65t + 32.5 km
The distance of Mr Ho is 80t km
In order for Mr Ho to be not less than 65 km ahead of Mr Lee,
80t - (65t + 32.5) >= 65
15t - 32.5 >= 65
15t >= 97.5
t >= 6.5 hours
It takes 6.5 hours for Mr Ho to be ahead of Mr Lee by at least 65 km
(2) (x + 50)/2 - x >= 10 ...(1)
x + (x + 50) >= 60 ... (2)
(1) => x + 50 - 2x > = 20
-x >= -30
x <= 30
(2) => 2x + 50 >= 60
2x >= 10
x >= 5
(a) The range of the amount with Ivan is $5 <= x <= $30
(b) The range of the amount with Tony is 5 + 50 <= x+50 <= 30 + 50
or $55 <= x + 50 <= $80

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