A cone penetration test on a coarse$-$grained soil with mean particle size $($D$50)$ of $0\mathrm{.}5$ mm produced a cone resistance $($qc$)$ of $80$ kg$/$cm$2$ and a friction ratio $($Rf$)$ of $1\%.($a$)$ Estimate the equivalent SPT N$60$ value using the qc$-$N$60$ relationship suggested by Kulhawy and Mayne $(1990)$