People with dyscalculia may lack any concept of a number. This means that what comes naturally to many people, is a constant struggle.
They have to learn even the most basic calculations by rote as you would a poem or a foreign language. This means that answering what is 5+5 involves remembering a fact rather than having an intuitive response. This means that they may not be very confident about their issues.
People with dyslexia may also struggle with mental calculations because of issues with working memory. They simply have a smaller buffer. But they will be fine if given more time or writing the numbers down.
Use physical aids like fingers, abacus or calculator. Also, allow more time for processing.