superpus07 wrote:

Machine A currently takes x hours to complete a certain job. Machine B currently takes y hours to complete the same job. If x=4y, by what percent will x have to decrease so that A and B together can complete the job in 3y/8 hours ?

A. 15%

B. 25%

C. 30%

D. 62,5%

E. 85%

Let y be 8

=> x = 32

let total units of work to be done is 32 units

so a does 1 unit/hr

and b does 4 unit/hr

now we are asked to complete the work in 3y/8 or 3*8/8 = 3 hrs

so we would need to complete 32/3 = 10.66 units/hr

right now we are doing 5 units, so a would actually need to do 6.66 units/hr

so a would complete all the work in 32/6.66 = 4.80 hrs

hence x is reduced by (32-4.8)/32 = 0.85 or 85%

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Illegitimi non carborundum.