Suppose that .625 mol HBr partially react with Li to give .278 mol H2. Determine the number of moles HBr remain. SHOW PROCESS, I am confused about how he reached that answer

Suppose that 625 mol HBr partially react with Li to give 278 mol H2 Determine the number of moles HBr remain SHOW PROCESS I am confused about how he reached tha class=