The speed of the motor boat along the river is 18 2/5 km / h, and the speed of the river is 2 3/4 km / h.

The speed of the motor boat along the river is 18 2/5 km / h, and the speed of the river is 2 3/4 km / h. Find the speed of the motorboat against the current of the river.

1) Let’s calculate the own speed of the motor boat:
1/8 2/5 – 2 3/4 = (18 – 2) + (2/5 – 3/4) = 16 + (8/20 – 15/20) = 16 – 7/20 = 15 + (1 – 7/20) = 15 + (20/20 – 7/20) = 15 + 13/20 = 15 13/20 km / h.

2) Next, we find out the speed of the motor boat against the river flow. To do this, subtract the speed of the river from the boat’s own speed:
15 13/20 – 2 3/4 = (15 – 2) + (13/20 – 3/4) = 13 + (13/20 – 15/20) = 13 – 2/20 = 13 – 1/10 = 12 + (1 – 1/10) = 12 + (10/10 – 1/10) = 12 + 9/10 = 12 9/10 km / h.
Answer: 12 9/10 km / h.