Tim and David are 20 km apart.If they walk towards each other, they will meet in 2 hours. If they walk towards the same direction and Tim
is behind David , Tim will catch up with David in 5 hours .Find their
speeds.
[Let x km/h be the speed of Tim and y km/h be that of David.]
極度困難的中二數學問題
2009-03-01 11:14 pm
回答 (2)
2009-03-01 11:32 pm
✔ 最佳答案
Distance travelled by Tim in 2 hours = 2x.Distance travelled by David in 2 hours = 2y.
so 2x + 2y = 20
x + y = 10 .................(1)
Distance travelled by David in 5 hours = 5y.
Distance travelled by Tim in 5 hours = 5x.
since Tim is 20 km behind David,
so 20 + 5y = 5x
x - y = 4.................(2)
(1) + (2) we get 2x = 14, x = 7 km/h.
y = 10 - x = 10 - 7 = 3 km/h.
2009-03-02 4:45 am
2(x+y)=20...(1)
x+y=10...(2)
5(x-y)=20...(3)
x-y=4...(4)
(2)-(4)
x+y-(x-y)=10-4
x+y-x+y=6
2y=6
y=3...(5)
Join (5) into (2)
x+3=10
x=7
x+y=10...(2)
5(x-y)=20...(3)
x-y=4...(4)
(2)-(4)
x+y-(x-y)=10-4
x+y-x+y=6
2y=6
y=3...(5)
Join (5) into (2)
x+3=10
x=7
收錄日期: 2021-04-25 22:39:42
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