Whenever we multiply two numbers under radicands (√), they are multiplied together under a single radicand.
√10 • √8 = √80
Simplify the radical by finding by factoring the number and looking for perfect squares:
80 = 5 * (2 * 2) * (2 * 2)
√80 = √5 * √16
√80 = √5 * 4
√80 = 4√5