Doris bought 1263 stickers because 1029+234=1263 and Doris bought 234 more stickers than Alex bought, and Alex had bought 1029 stickers.
Mamie bought 1456 stickers because 1263+193=1456 and Mamie bought 1456 stickers which is 193 more than Doris bought. Doris bought 1263 which we have figured out in the last problem.