Question

If two brothers work together they can change the oil in the family car in three minutes. If it takes the older brother 12 minutes to change the oil alone, how long does it take for the younger brother to change the oil?

Answer 1

(O * Y)/(O + Y) = 3
(12 * Y)/(12 + Y) = 3
solve for Y

Answer 2

Y = Young
O = old

Answer 3

In these kind of sums always find the work done in unit time
in 1 min both can do only 1/3 part of work
but in 1 min older does alone 1/12 part of work
thus in 1min younger does 1/3-1/12=(4-1)/12=3/12 =1/4 of work
thus it takes 4mins for the younger to change the oil