Explain why the "add a decimal and zero rule" work?

Need solving this second acceleration problem.

What is the lcm of 16 and 40 ? 24 and 40 ?

Prove the divisibility of the following numbers: 25^9 + 5^7 is divisible by 30.also, read as (25 to the power of 9) + (5 to the power of 7) is divisible by 30. blank x 30what is the blank? ( it should be expressed in exponent form)