RON MADE A MISTAKE CASTING A SPELL, AND 25 FROGS START TO APPEAR EA...

7. Ron made a mistake casting a spell, and 25 frogs start to appear each second in the classroom. After 10 seconds, Hermione cast another spell, and 35 frogs start to disappear as 25 frogs appear each second. For how many seconds was there at least one frog in the classroom?