\(\eqalign{
& a)\,\,{{ - 1} \over {21}} + {{ - 1} \over {28}} \cr
& = {{( - 1).4} \over {21.4}} + {{( - 1).3} \over {28.3}} \cr
& = {{ - 4} \over {84}} + {{ - 3} \over {84}} = {{ - 7} \over {84}} = {{ - 1} \over {12}} \cr} \)
\(\eqalign{
& b){{ - 8} \over {18}} - {{15} \over {27}} = {{ - 8} \over {18}} + \left( { - {{15} \over {27}}} \right) \cr
& = {{ - 4} \over 9} + {{ - 5} \over 9} = {{ - 9} \over 9} = - 1 \cr} \)
\(\eqalign{
& c)\,\,{{ - 5} \over {12}} + 0,75 = {{ - 5} \over {12}} + {3 \over 4} \cr
& \, = {{ - 5} \over {12}} + {9 \over {12}} = {{ - 5 + 9} \over {12}} \cr
& \, = {4 \over {12}} = {1 \over 3} \cr} \)
\(\eqalign{
& d)\,\,3,5 - \left( { - {2 \over 7}} \right) = 3,5 + {2 \over 7} \cr
& = {{35} \over {10}} + {2 \over 7} = {7 \over 2} + {2 \over 7} = {{49} \over {14}} + {4 \over {14}} \cr
& = {{49 + 4} \over {14}} = {{53} \over {14}} = 3{{11} \over {14}} \cr} \)
