Direct and indirect proportion Qs

Chinn_asama wrote:Hi

Can anyone explain how to solve the below direct and indirect proportion problems?

1. If 36 men can do a piece of work in 25 days, in how many days will 15 men do it?

2. If 20 men can build a wall 112m long in days, what length of a similar wall can be built by 25 men in 3 days?

Thanx

I do them logically. Just start with the unknown on right and multiply divide as required.

36 men need -------- 25 days
1 man needs--------- 25*36 (less men will need more days, hence multiply)
15 men need------- 25*36/15 = 60 days ( more men (change from 1 to 15) will need less days, hence divide.)

For the second, using the same idea as above....
The number of days in the question is missing, I am going to assume 6 days.

Then:

20 men in 6 days will make ------ 112m long wall
1 man in 6 days -------- 112/6 (less men will build less, hence divide)
1 man in 1 day --------- 112/(20*6) ( again less time means smaller wall. Divide)
25 men in 1 day ----- 25* 112/(20*6)
25 men in 3 days ------ 25*3*112/(20*6).

Cheers

Plug in a rate for EACH WORKER.

1. If 36 men can do a piece of work in 25 days, in how many days will 15 men do it?

Let the rate per man = 1 unit per day.
Rate for 36 men = 36 units per pay.
In 25 days, the number of units produced by 36 men = 36*25 units.
Rate for 15 men = 15 units per day.
Time for these 15 men to produce the same number of units = (36*25)/15 = 60 days.

2. If 20 men can build a wall 112m long in 6 days, what length of a similar wall can be built by 25 men in 3 days?

Let the rate per man = 1 unit per day.

Rate for 20 men = 20 units per day.
Number of units produced by 20 men in 6 days = 20*6 = 120.

Rate for 25 men = 25 units per day.
Number of units produced by 25 men in 3 days = 75.

To determine the number of METERS produced by the 25 men, set up the following proportion:
(Work for 25 men)/(work for 20 men) = (x meters)/(112 meters)
75/120 = x/112
x = 70.

sorry. In the 2nd question it is 6 days.