1) ((187+325+b)-176)+305=901
((187+325)-176)+305+b=901
(512-176)+305+b=901
336+305+b=901
641+b=901
b=901-641
b=260
187+325+260=772-176=596+305=901
2) (((1905-n)+236)+187)-584=20
((1905+236+187)-584)-n=20
(2328-584)-n=20
1744-n=20
n=1744-20
n=1724
1905-1724=181+236=417+187=602-584=20