Please help me on this one! I don’t understand

Using only the digits 5, 6, 7, 8, how many different three digit numbers can be formed if no digit is repeated in a number?

Aliter of water contains about 3.35 x 10^25 molecules. a certain river discharges about 2.9x 10^8 l of water every second. about how many molecules does the river discharge every minute? write your answer in scientific notation

Find the value of x to the nearest tenth. a. 4.5 b. 5.4 c. 6.3 d. 7.2