The answer is 7.
f(a) = 2a-1, 3f(a) = 3(2a-1) = 15 -> a=2 so f(2a)=f(4)=7
Answer:
5 (5x1) , -10 -(5x2), 20(10x2), -40 -(20x2), 80(40x2)
so the next number should be previous number in the sequence multiplied by 2 and a negative sign.
-`(80x2) = -160
The 5 number that have 3 ,5 and 7 as prime number are 23, 31, 37, 43, 53. and if you want there 57,47, 103.
