Let x be the morning sales.
Afternoon sales = x + 50
Evening sales = 3(x + 50)
Total sales = x + (x + 50) + 3(x + 50) = 1000
Equation needed = 1000 - x - (x + 50) = 3(x + 50)
To find x:
x + (x + 50) + 3(x + 50) = 1000
x + (x + 50) + 3x + 150 = 1000
5x + 200 = 1000
5x = 1000 - 200
5x = 800
x = 800 / 5 = 160
Hence, evening sales:
3(x + 50) = 3x + 150 = 3(160) + 150 = 630
I hope I have managed to help you!!