Please help I’ll mark you as brainliest if correct!

Prove that the sum of two consecutive exponents of the number 5 is divisible by 30.if two consecutive exponents are 5n and 5n + 1, then their sum can be written as 30.

On a string instrument, the length of a string varies inversely as the frequency of its vibrations. an 11-inch string has a frequency of 400 cylces per second. find the frequency of a 10-icnch string.

Jenisey has $24 to spend on seven pencils. after buying them she had $10. how much did each pencil cost.?