what does rivka mean when she says it is brutal arithemetic

"It is a brutal arithmetic. But I - I am alive. You are alive. As long as we breathe, we can see and hear. As long as we remember, all those gone before are alive inside us."

Rivka expresses her thankfulness for every second that she is alive and breathing. This scene unfolds as Esther expresses her dismay at Rivka's statement that being alive in that moment is enough. Esther cannot understand this. She tells Rivka that she has lost numerous relatives already. What can she possibly be thankful for? With the threat of death constantly looming over them, Rivka's expression suggests a new appreciation for life even for the most mundane or even terrible moments. At least, Rivka reminds them, they are still alive. There are others who have not been so lucky. But, because they are alive those who are gone are not forgotten. They can live on in the memories of the living. Rivka admits that it is a harsh reality, a "brutal arithmetic", but as long as they are alive there is hope and hope is their greatest asset against the Nazi onslaught.