唔識做功課13 - Ans.fyi 參考答案

唔識做功課13

2013-08-01 3:44 am

1 .When travelling over the same distance, if the speed is increased by 25%,what is the percentange change in the time taken?

2 .The number of boys and grils in a school are in the ratio 7:3. If the number of boys is decreased by 10% and that of grils is increased by 40%, find

(a) the new ratio of the numbers of boys and girls in the school,

(b) the percentage chance in the total number of students.

回答 (1)

2013-08-01 4:32 am

✔ 最佳答案

1.Since distance=speed*time, d=v*tOriginally, t=d/vNow, d keeps unchanged, v is increased by25% to 1.25*vSo, new time=d/(1.25*v)=0.8*(d/v)=0.8*tPercentage change=(0.8*t-t)/t*100%=-20% Answer = -20% (or is decreased by 20%) 2.(a)Let the number of boys be 7*N, for somepositive number N.Then the number of girls is 3*N.New ratio= (7*N)*(1-10%):(3*N)*(1+40%)=6.3*N:4.2*N=(6.3*N)/(4.2*N)=3/2=3:2Answer=3:2 2.(b)Percentage change={[(7*N)*(1-10%)+(3*N)*(1+40%)]-(7*N+3*N)}/(7*N+3*N)*100%=(10.5*N-10*N)/(10*N)*100%=[(10.5-10)*N]/(10*N)*100%By simplification, cancelling the common factorN=0.5/10*100%=+5%Answer=+5%

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