Given data:
| Downstream distance | Upstream distance | Time |
|---|
| 33 km | 35 km | 8 hours |
| 44 km | 28 km | 8 hours |
| 55 km | 14 km | |
Calculation:
Let the speed of river be “x” and speed of boat be “y”.
Effective downstream speed = x + y
Effective upstream speed = x – y
x+y33+ x−y35 = 8 (Eq. 1)
x+y44 + x−y28= 8 (Eq. 2)
Subtracting equation 1 from equation 2-
⇒ x+y44 - x+y33 + x−y28 - x−y35= 0
⇒ x+y44 - x+y33 = x−y35 - x−y28
⇒ x+y11 = x−y7
⇒ 11 × (x - y) = 7 × (x + y)
⇒ 11 x - 11y = 7x + 7y
⇒ 4x = 18y
⇒ x = 29y.
Putting the value of "x" in equation 1-
⇒ 29y+y33 + 29y−y35 = 8
⇒ 11y66 + 7y70 = 8 or y6 + y10 = 8
⇒ y = 2
Substituting the value of "y" in the value of "x" = 9
∴ Time required for 55 km downstream and 14 km upstream distance = x+y55+ x−y14
⇒ Time required for 55 km downstream and 14 km upstream distance = 9+255+ 9−214= 5 + 2 = 7 hours
The total time required for the distance to be traveled is 7 hours.
Shortcut TrickHere, according to the question,
⇒ 33/(x + y) + 35/(x - y) = 8 & 44/(x + y) + 28/(x - y) = 8
As the number of required hours is a complete number hence
The values of (x + y) & (x - y) should divide the respective numbers
For Downstream, that is 33 and 44 both are multiples of 11
and for Upstream, that is 35 and 28 both are multiples of 7
By value putting method,
⇒ 33/11 + 35/7 = 3 + 5 = 8 & 44/11 + 28/7 = 4 + 4 = 8
⇒ x + y = 11 & x - y = 7
Required time,
⇒ 55/11 + 14/7 = 5 + 2 = 7 hours
∴ The total time required for the distance to be traveled is 7 hours.