Make a sketch of a scale.


Put/draw  a sandwich, a cup of tea and a samosa in the left side/cup of the scale.
Put/draw  Rs 28.50 on the right side/cup of the scale.

Next step is 

   1)  to put Rs 11.50 to the left cup and to add Rs 11.50 to the right cup of the scale.

   2) On the left side, to unite this Rs 11.50 with the cup of tea, and

   3) to replace the united (cup of tea + Rs 11.50) with the other equivalent sandwich.

Now you have two sandwiches and the samosa on the left cup of your scale, as well as 28.50 + 11.50 = Rs 40 on the right cup.


Next step is to make similar operations with . . . . 


I will omit the details. They must be clear to you.


So, the conception of a "scale" will replace the conception of "equation" for your child.


It works perfectly !  Checked many times ! . . .