Question

A and B can fill a cistern in 4 hours. A and C can fill

Answer 1

Answer:

20 hours

Step-by-step explanation:

We can work easier if we think in terms of an hour

the first equation that we can write is

A + B = 1/4 (its meaning is that in an hour A and B fill 1/4 of the cistern)

the second equation we can write is

A + C = 1/5 (its meaning is that in an hour A and C fill 1/5 of the cistern)

the third equation that we can write is

B = 2C  (its meaning is that in an hour B fill 2 times C the cistern)

now we can substitute the value of B in the first two equations

A + 2C = 1/4

A + C = 1/5

A = 1/5 - C

1/5 - C + 2C = 1/4

C = 1/4 -1/5

C = (5-4)/20

C = 1/20 (this means that in an hour C will fill 1/20 of the cistern, so it takes 20 hours to fill the entire cistern)

A = 1/5 - 1/20

A = (4-1)/20 = 3/20

B = 1/20 *2 = 1/10