It is a defect in a measuring device (vernier calliper and screw gauge) and it is caused by an incorrect position of the zero points.
Zero correction means to make the adjustment in such a way that the measuring instrument shows zero when the input is zero.
Vi = 30 ms-1
Vf = 15 ms-1
Change in velocity = Vf - Vi
= 15 ms-1 - 30 ms-1
= -15 ms-1
Time taken = 5 s
a = change in velocity / time interval
a = -15 ms-1 / 5 s
a = -3 ms-2
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