1) (5/12-3/18)= 5/12-1/6=5/12-2/12=3/12
2) 3/12*4=12/12=1
3) 1-5/4=-1/4=-0,25
Ответ: -0,25
1 9/17*(х-0,4)=4 1/5*13/21
26/17х-52/85=21/5*13/21
26/17х-52/85=13/5
26/17х=13/5+52/85
26/17х=273/85
х=273/85:26/17
х=273/85*17/26
х=21/10
х=2,1
420 = 2 * 2 * 3 * 5 * 7 * 1
140 = 2 * 2 * 5 * 7 * 1
180 = 2 * 2 * 3 * 3 * 5 * 1
НОК = 2 * 2 * 3 * 3 * 5 * 7 * 1 = 4 * 9 * 35 * 1 = 1260