To subtract fractions, you must have a common denominator.

First, find the least common denominator (LCD) for each pair of fractions.

For 3/50 and 18/25, the LCD is 50.

3/50 - 18/25
= (31)/(501) - (182)/(252)
= 3/50 - 36/50
= -33/50

For 43/42 and 10/31, the LCD is 42.

43/42 - 10/31
= (431)/(421) - (102)/(312)
= 43/42 - 20/62
= 43/42 - 10/31
= (4331)/(4231) - (1042)/(3142)
= 1333/1302 - 420/1302
= 913/1302

Therefore, the solution is:
3/50 - 18/25 = -33/50
43/42 - 10/31 = 913/1302