1. Initial ratio: Milk : Water = 3 : 4
   Total mixture = 35 liters → Total parts = 3 + 4 = 7

2. Quantities in mixture:

   • Milk = $35 × \frac{3}{7} = 15$ liters

   • Water = $35 × \frac{4}{7} = 20$ liters

3. Let x = milk to be added
   New ratio Milk : Water = 1 : 1 → $\frac{15 + x}{20} = 1$

4. Solve for x:
   $15 + x = 20 \implies x = 5$

Answer: 5 liters of milk should be added.

Question: A swimmer swims in a river against the current and takes 30 minutes to swim 2 km. With the current, he takes 20 minutes to swim the same distance. What is his speed in still water?

Solution:
Given that,
distance covered= 2 km 
time consumed = 30 min = 30/60 hr = 1/2 hr
∴ speed of the swimmer against current (upstream) = 2/(1/2) = 4 km/hr

Again,
With the current,
distance covered = 2 km
time consumed= 20 min = 20/60 hr = 1/3 hr
∴ speed of the swimmer with current (downstream)= 2/(1/3) = 6 km/hr

∴ speed in still water = (speed in downstream + speed in upstream)/2
 = (6 + 4)/2
=10/2
= 5
So, the swimmer’s speed in still water is 5 km/h

Question: A ferry can carry 25 trucks or 40 motorcycles at a time. If there are 15 trucks on the ferry, how many motorcycles can be loaded onto it?

Solution:
Here,
25 trucks = 40 motorcycles
∴ 1 trucks = 40/25 = 8/5 motorcycles
∴ 15 trucks = = (8 × 15)/5 = 24 motorcycles

∴ Required number of motorcycles = 40 - 24 = 16

So the ferry can carry 16 more motorcycles along with the 15 trucks.