Two beepers beeps at 4 and 9 seconds respectively then in an hour how many times will they beep?

First, let's see how many times it beeps in one minute. Firstly, you want to double the number after every beep until it reaches 60.

Beeper 1:

4 seconds: 1
8 seconds: 2
12 seconds: 3
16 seconds: 4
20 seconds: 5
24 seconds: 6
28 seconds: 7
32 seconds: 8
38 seconds: 9
42 seconds: 10
46 seconds: 11
50 seconds: 12
54 seconds: 13
58 seconds: 14

Beeper 1 total: 14
Leftover seconds: 2

Beeper 2:

9 seconds: 1
18 seconds: 2
27 seconds: 3
36 seconds: 4
45 seconds: 5
54 seconds: 6

Beeper 2 total: 6
Leftover seconds: 6

Leftover seconds from Beepers 1 & 2: 2 + 6 = 8
Since that can't quite make 9, you must make two "4 seconds" out of Beeper 1, bringing the overall Beeper 1 answer is 16 beeps.

Overall Answers:

Beeper 1 grand total: 16
Beeper 2 grand total: 6

16 + 6 = 21 beeps a minute

Now we want to multiply 21 x 60. I reckon you know how to show your work to get this simple answer. Multiply that and that's your overall answer.

Assuming you've done that by now, your answer should come to a grand total of 1260 beeps per hour between the two.