please check my answers

1.which is smallest 5/8,3/4,9/16,7/8 =9/16
2.subtract 6 3/10-4 11/15=1 17/30
3.estimate 6 1/8* 7 4/5=48
4.lcm of 20,32=160
jon spent 1/3 of his study time doing math and 2/7 preparing for history exam. what fraction of his study time remains? = 8/21
5.lcm of 10,15,25=150
6. add 3/11+9/11+1/11=1 2/11