If the average weight of a class of 20 students decreased by 0.25 kg when a boy weighing 48 kg was replaced by another boy weighing A kg, find the value of A.

To find the value of A, follow these steps:

1. Calculate the total weight of the class before the replacement.

   • Let the average weight of the class before the replacement be $W$ kg.
   • Therefore, the total weight of the class before the replacement is $20W$ kg.

2. Calculate the new average weight after the replacement.

   • The average weight decreased by 0.25 kg, so the new average weight is $W - 0.25$ kg.
   • Therefore, the total weight of the class after the replacement is $20(W - 0.25)$ kg.

3. Set up the equation for the total weight before and after the replacement.

   • The total weight before the replacement is $20W$ kg.
   • The total weight after the replacement is $20(W - 0.25)$ kg.
   • The difference in total weight is due to the replacement of the boy weighing 48 kg with the boy weighing $A$ kg.

4. Write the equation for the total weight after the replacement: $20(W - 0.25) = 20W - 5$

5. The difference in total weight is equal to the weight of the boy who left minus the weight of the boy who joined: $20W - 5 = 20W - 48 + A$

6. Simplify the equation to find $A$: $-5 = -48 + A$ $A = 48 - 5$ $A = 43$

Therefore, the value of $A$ is 43 kg.