3 вариант. Вычислите: 1) 1/4 + 2/3; 2) 5/7 − 1/4; 3) 1/3 · 6/7; 4) 3/10 : 1/5; 5) 1 3/4 + 2 5/6; 6) 1 1/4 − 2/3; 7) 2 1/2 · 3 4/9; 8) 4 1/6 : 2 2/3; 9) 1/2 + 4; 10) 4 − 2/5; 11) 3 · 2/3; 12) 3/8 : 7.

1. \(\frac14+\frac23=\frac3{12}+\frac8{12}=\frac{11}{12}\).
2. \(\frac57-\frac14=\frac{20}{28}-\frac7{28}=\frac{13}{28}\).
3. \(\frac13\cdot\frac67=\frac6{21}=\frac27\).
4. \(\frac3{10}:\frac15=\frac3{10}\cdot5=\frac32=1\frac12\).
5. \(1\frac34+2\frac56=\frac74+\frac{17}{6}=\frac{21}{12}+\frac{34}{12}=\frac{55}{12}=4\frac7{12}\).
6. \(1\frac14-\frac23=\frac54-\frac23=\frac{15}{12}-\frac8{12}=\frac7{12}\).
7. \(2\frac12\cdot3\frac49=\frac52\cdot\frac{31}{9}=\frac{155}{18}=8\frac{11}{18}\).
8. \(4\frac16:2\frac23=\frac{25}{6}:\frac83=\frac{25}{6}\cdot\frac38=\frac{25}{16}=1\frac9{16}\).
9. \(\frac12+4=4\frac12\).
10. \(4-\frac25=3\frac35\).
11. \(3\cdot\frac23=2\).
12. \(\frac38:7=\frac38\cdot\frac17=\frac3{56}\).

Ответ: 1) 11/12; 2) 13/28; 3) 2/7; 4) 1 1/2; 5) 4 7/12; 6) 7/12; 7) 8 11/18; 8) 1 9/16; 9) 4 1/2; 10) 3 3/5; 11) 2; 12) 3/56.