(12-13):(13-14)*(14-15):(15-16)=

1/2 - 1/3 = 3/6 - 2/6 = 1/6
1/3 - 1/4 = 4/12 - 3/12 = 1/12
1/6 : 1/12 = 1/6 * 12 = 2
1/4 - 1/5 = 5/20 - 4/20 = 1/20
2* 1/20 = 1/10
1/5 - 1/6 = 6/30 - 5/30 = 1/30
1/10 : 1/30 = 1/10 * 30 = 3 
Отв 3