a) \({{1,28} \over {3,15}} = {{1,28.100} \over {3,15.100}} = {{128} \over {315}} \)

b) \({2 \over 5} \div 3{1 \over 4} = {2 \over 5} \div {{13} \over 4} = {2 \over 5}.{4 \over {13}} = {8 \over {65}} \)

c) \(1{3 \over 7} \div 1,24 = {{10} \over 7} \div {{124} \over {100}} = {{10} \over 7}.{{100} \over {124}} = {{1000} \over {868}} = {{250} \over {217}} \)

d) \({{2{1 \over 5}} \over {3{1 \over 7}}} = {{{{11} \over 5}} \over {{{22} \over 7}}} = {{11} \over 5}.{7 \over {22}} = {7 \over {10}} \)