What is 4 over 5 -1 over 6 i have 30 -30 but i dont know how to find what goes 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.

How many kilograms of a 5% salt solution and how many kilograms of a 15% salt solution must be mixed together to make 45kg of an 8% salt solution?

Jimmy can run 3.5 miles in 20 minutes. how far can ne run in one hour and ten minutes?