Mesophere plus thermosphere reflects radio waves. what is it called

Surplus discretionary travel direct money is what's available after paying for the necessities of life

Brian's school locker has a three-digit combination lock that can be set using the numbers 5 to 9 (including 5 and 9), without repeating a number. the probability that the locker code begins with a prime number is %. the probability that the last digit of the locker code is an odd number is %.

Given: circle a externally tangent to circle b. how many possible common tangents to circles a and b can exist? 2 3 4