Step-by-step explanation:
x = mass of apples
y = mass of strawberries
z = mass of the box
x = 3y
x + z = 16240
y + z = 6350
let's subtract the second from the first equation
x + z = 16240
- y + z = 6350
-----------------------
x - y + 0 = 9890
using the first identity in that result gives us
3y - y = 9890
2y = 9890
y = 4,945 g
x = 3y = 3×4945 = 14,835 g = the mass of the apples