Let the maximum marks which can be obtained in the subjects P, Q, R and S be $40p, 50q, 60r$ and $80s$ respectively.
From the given information we can conclude that:
$15p≥20q$, $20p≥10q$, $25p≥15q$, $10p ≥18q$, $25p≥32q$ and $30p≥ 12q$
$⇒ p ≥ 1.8q$ -------- (i)
$15p≥8r$, $20p≥16r$, $25p≥20r$, $10p≥24r$, $25p≥10r$ and $30p≥20r$
$⇒ p≥2.4r$ -------- (ii)
$15p≤25s$, $20p≤15s$, $25p≤20s$, $10p≤30s$, $25p≤10s$ and $30p≤35s$
$⇒ p≤ 0.4s$ -------- (iii)
Given that the value of $X$ is 91.
Maximum possible marks that can be obtained by A, B, C, D, E and F in subject Q are 70, 35, 52.5, 63, 112 and 42 respectively.
So, none of B, C, D and F can obtain more marks than 65 in subject Q.