Let at the time $t$ hrs , they meets
Distance covered by that move from Mumbai, say $x= 60t$ Km
Distance covered by that move from Pune, say $y= 40t$ Km
Distance between Mumbai and Pune $=x+y =60t+40t=100t$ Km
one having more speed with cover more distance in same time.
so, $x=y+20$
$60t=40t+20$
$t= 1$hr
Distance between Mumbai and Pune $= 100 \times 1 = 100$Km