а) 7/15-3/10=2×7/ 2×15-3×3/3×10=14/30-9/30=14-9/30=5/30=1×5/6×5=1/6
б)
5/42 : 6/7 =5/42 ×7/6 =5×7/ 42×6=35/252=5×7/36×7=5/36
