$($a$)$ Estimate the fugacity of isobutylene as a gas at $280C$ and $100$ bars by using the following:

$($i$),2^{nd}$ virial coefficient

$($ii$)$ Lee$/$Kesler correlation

$($iii$)$ Which method is more accurate and why?

$($b$)$ If the pressure is reduced to $20$ bar, what would be the effect on the fugacity coefficient in part $($a$),$ it will be increased or decreased and why?