Try different possibilities of running these two threads and primpt processes to get the desired result.
For example 900 as outcome is possible when T1 reads A=1000, then primpted and T2 reads A=1000. Primpt T2 run T1 3 times and make A = 1300. Now run T2 for 2 times, Since T2 has read A =1000 and not 1300 so after 2 iterations it will make A=900.
Similary you can try for other possibilities also.