Explanation

Let's denote the present age of A as 3x and B as 4x.

Step 1: Express their ages 10 years ago

A's age 10 years ago = 3x - 10

B's age 10 years ago = 4x - 10

Step 2: Formulate the equation based on the given information

A was half of B's age 10 years ago:

3x - 10 = 1/2 * (4x - 10)

Step 3: Solve the equation

3x - 10 = 2x - 5

3x - 2x = -5 + 10

x = 5

Step 4: Find their present ages

A's age = 3x = 3 * 5 = 15

B's age = 4x = 4 * 5 = 20

Step 5: Calculate the total of their present ages

Total age = 15 + 20 = 35