Can Somebody help me with this problem???

I am having trouble to come with a approach and code for this problem, can somebody help me out?
Here is the question

Antas has a garden where plants are planted in a row. Each plant is represented by a character which can be a letter or a number. However, his father does not like the design and he wants to change it to a new design. This new design is also represented by characters. To change the garden, he can take the following actions:

1. Plant a sapling
2. Uproot a sapling
3. Replace the plant with another

Each of the actions have an associated cost which are as follows:

• Planting - \$100
• Uproot - \$100
• Replacing - \$100

Help him find the minimum cost for changing the garden to the new design.

Input Format

``````The first line contains t which gives the number of rows in the garden. Then for each row, there are two lines which represent the following:
1. The first line contains a string of characters that represent the old design.
2. The second line contains a string of characters that represent the new design.
Output Format
The output contains r lines where ith line contains the minimum cost of changing ith row from old design to new design.
Constraints
1 <= r <= 10000
1 <=n <= 100, where n is the length of the string of characters representing the plants

Sample Input
2
mobile
phone
help
all
d

Sample Output
\$500.00
\$300.00
``````

Explanation

``````For the first row, it will need to plant one ‘p’ plant which costs \$100, then replacing ‘m’ to ‘h’, which

costs \$100, no change for plant ‘o’, replacing ‘b’ with ‘n’ which costs \$100, then uprooting ‘i’ and ‘l’

which costs \$100 each. In  total   ,  the  minimum   cost  for changing  row  1 to  new design  needs

\$500.``````