10. a) 4/10<_ Z/10<_7/10
=> 4<_Z<_7
=>Z=4,5,6,7
b)7/5<_7/Z<_7/5
=>5<_Z<_5
=>Z=5
c)4/9<_4Z/9<_16/9
=>4<_4Z<_16/÷4
=>1<_Z<_4
=>Z=1,2,3,4