OK,
The 74193 is a 4 stage Binary counter so that it divides by 16, what you need to do is divide the 50 by 10 in the first 74193, so thats 5 pulses/sec out and then divide the 5 pulses by 5 in the second 74193, that will give a 1 second pulse.
So the NAND gates are used to detect when the first 74193 has reached a count of 10,,, reset itself and clock the second 74193 on by one pulse.
In the second 74193 the NAND gates will have to detect when the count gets to 5,, reset itself and output a 1 second clock pulse.
Are you able to follow that explanation.?