Could somebody help me with these questions?

1) There are some pupils in 6A and 6B classrooms. If 15 pupils from 6A were to move to 6B, 6B would have thrice as many pupils as 6A. If, instead, 15 pupils from 6B were to move to 6A, 6A would have twice as many pupils as 6B. How many pupils are there in the 6A classroom?

2) Xue Ying and Brian have some stickers between them. If Xue Ying gave 42 stickers to Brian, Brian would have four times as many stickers as Xue Ying. If, instead, Brian gave 42 stickers to Xue Ying, Xue Ying would have twice as many sticker as Brian. How many stickers does Xue Ying have?

3) Siti and Aloysius have some money between them. If , Siti gave $77 to Aloysius, Aloysius would have four times as much money as Siti. If, instead, Aloysius gave $77 to Siti, Siti would have thrice as much money as Aloysius. How much money does Siti have?