What is the next fraction in this sequence 3/10, 1/4, 1/5, 3/20,

1/10 is the next fraction in the sequence. If you multiply the numerator and denominator by a common factor to give each fraction a denominator of 20, the pattern goes 6/20, 5/20, 4/20, 3/20. The next term would be 2/20 which equals 1/10.