Student or Learner
When putting more credit into a mobile phone account or a public travel card, I oftem meet such kind of instruction by the that you can only add the amount of 50, 100, 150, 200 or the like .
My question is that how to decribe this kind of instruction in a formal way.
You can only put the USD50 or its integer multiple ?
One of my friend who is a greenhand selling the metallic sheet to overseas market also has meet this quesiton because the width of his products may be 100mm, 200mm, 300mm etc.
But he is confused on how to express this in a formal way instead of listing