Question

explain why 7 x 11x 13 + 13 is a composite number

Answer 1

In 7x11x13+13, By taking 13 common, We get,  13((7×11)+1)           Or, 13(77+1)           Or, 13×78. Clearly, 7×11×13+13 has two other factors(13 & 78) other than 1. Since a composite number has more than two factors other than 1 and itself, 7×11×13+13 is composite.      :)