Disposed as she then was to calculate upon that vague basis which allows the subtraction of one sum from another without any perceptible diminution, she was happy.
пример на вычитание.
en (матем.)
As a trivial example, one student may have scored five out of ten on his arithmetic test by answering the five additions correctly and the five subtractions incorrectly while his/her friend is given the same score for precisely the converse performance!
