For easy estimation, option A = nearest hundred,B = nearest ten, C = nearest hundred and D= nearest thousand.
3529 and 3689
3529 to the nearest hundred = 3500 and,
3689 to the nearest hundred = 3700
∴ Required sum = (3500+3700) = 7200
33 and 99
33 to the nearest ten = 30 and,
99 to the nearest ten = 90
∴ Required sum = (30+90) = 120
120777 × 523
777 to the nearest hundred = 800 and,
523 to the nearest hundred = 500
∴ Required sum = (800+500) = 1300
1300495420 and 123512
495420 to the nearest thousand = 495000 and,
123512 to the nearest thousand = 124000
∴ Required sum = (495000 + 124000 ) = 619000
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