Based on the given information, you have:
Steven's brother has 350 stamps
Steven's sister has 1,400 - 350 = 1,050 stamps
To determine the associated percent, proceed as follow:
For Steven's brother:
(x/100)1,400 = 350
x = 350(100/1,400)
x = 25
For Steven's sister:
(x/100)1,400 = 1,050
x = 1,050(100/1,400)
x = 75
a. Steven's brother has 25% of the stamps
b. Steven's sister brother has 75% of the stamps