The Timing....!

Given that all the electronic devices beeped together at 12 noon....

Moreover, the first device ( $f_{1}$ ) beeps every 30 minutes, the second device ( $f_{2}$ ) beeps every 60 minutes, the third device ( $f_{3}$ ) beeps every 90 minutes while the fourth device ( $f_{4}$ ) beeps after every 105 minutes.....

Thus what we got to find is the L.C.M of 30, 60, 90 and 105...

30 = $2 \times 3 \times5$

60= $2 \times2 \times3 \times5$

90= $2 \times3 \times3 \times5$

105= $3 \times 5 \times7$

Therefore, L.C.M. = $2 \times2 \times3 \times3 \times5 \times7$

= $\boxed{1260}$

but 1260 is in minutes, so divide it by 60 and you will get $\boxed{21}$ hours....

Thus add 21 hours to 12:00 pm(noon) thus you will get $\boxed{9 A.M}$

it should be 9:00 PM, i think, correct if im wrong?