For the first equation: 20a x 15a = 5;4
Solving it:
300a^2 = 5;4
Dividing both sides by 300: a^2 = 5;4 / 300
Taking the square root of both sides: a = +/- sqrt(5;4 / 300)

For the second equation: 25b x 5b = 4;12
Solving it:
125b^2 = 4;12
Dividing both sides by 125: b^2 = 4;12 / 125
Taking the square root of both sides: b = +/- sqrt(4;12 / 125)

For the third equation: 8c x 12c = 124;3
Solving it:
96c^2 = 124;3
Dividing both sides by 96: c^2 = 124;3 / 96
Taking the square root of both sides: c = +/- sqrt(124;3 / 96)