Highway Engineering - Engineering

Q1:

Minimum number of 50 kg cement bags per cubic metre of concrete for a mix corresponding to crushing strength 280 kg/cm2 at 28 days, are

A 5.0

B 6.5

C 7.0

D 7.5

E 8.0

ANS:B - 6.5

@N.K.Varsani.

How is it coming 6.5? Please explain.