1.
1.3 : 1/5 = 13/10 : 1/5 = 13/10 * 5/1 = 13/2 = 6.5
16 : 0.8 = 16 : 8/10 =16 * 10/8 = 20
20 > 6.5
16: 0.8 > 1.3 : 1/5
2.
приводим к общему знаменателю
7/x - x/5 = 35/5x - x²/5x = (35-x²)/5x
3.
5√27 / √12 = 5 √3²*3 / √2²*3 = 5*3√3 / 2√3 = 15/2 = 7.5