6. Two women, Sita and Gita working separately can mow a field in 8 and 12 hrs respectively. If they work in stretches of one hours alternatively, Sita beginning at 9 a.m., when will the mowing be finished?
In the first four Sita mows 1/8 of the field.
In the second hour Gita mows 1/12 of the field.
∴ In first 2hrs (1/8 + 1/12 = 5/24) of the field is mowed.
∴ In 8hrs 5/24 × 4 = 5/6 of the field is mowed.
Field remained to be mowed = 1 - 5/6 = 1/6
In the 9th hour Sita mows 1/8 of the field.
∴ Gita will finish the mowing of (1/6 - 1/8) = 1/24 of the field in (1/24 ÷1/12) or 1/2 of an hour.
∴ The total time required is (8+1+1/2) or 9(1/2)hrs.
Thus, the work will be finished at 6.30 pm.
In the first four Sita mows 1/8 of the field.
In the second hour Gita mows 1/12 of the field.
∴ In first 2hrs (1/8 + 1/12 = 5/24) of the field is mowed.
∴ In 8hrs 5/24 × 4 = 5/6 of the field is mowed.
Field remained to be mowed = 1 - 5/6 = 1/6
In the 9th hour Sita mows 1/8 of the field.
∴ Gita will finish the mowing of (1/6 - 1/8) = 1/24 of the field in (1/24 ÷1/12) or 1/2 of an hour.
∴ The total time required is (8+1+1/2) or 9(1/2)hrs.
Thus, the work will be finished at 6.30 pm.