№1.
5х+6х=77
11х=77
х=77÷11
х=7
5 × 7 + 6×7=77
35 + 42 = 77
77=77
17а -15а = 80
2а=80
а= 80 ÷2
а=40
17×40 - 15×40=80
680 - 600 =80
80=80
5к+7к - 11 = 98
12к=98+11
12к=109
к = 109÷12 = 109/12
к = 9 1/12
5 × 9 1/12 + 7× 9 1/12 - 11 =98
5/1 × 109/12 + 7/1 × 109/12 - 11 =98
(545 +763)/12 - 11 =98
1308/12 - 11 = 98
109 - 11=98
98=98
12с +8с -20 =43
20с = 43+20
20с= 63
с= 63/20 = 3 3/20
с= 3,15
12×3,15 +8× 3,15 - 20 =43
37,8 +25,2 - 20 =43
63 -20 = 43
43=43
№2.
1)
45а - 40а = 5а
а=7 ⇒ 5×7 = 35
2)
19b+21b+15 = 40b +15
b= 3 ⇒ 40×3 + 15 = 120 +15 = 135