First, let's make an attempt to find the unit rate:
42/60 = 14/20
14/20 = 7/10
7/10 is close enough, because 40 is a multiple of 10.
Because 40/10 = 4, we'll have to multiply both sides of the fraction by 10:
7*4/10*4
28/40
Miss Sim will travel 28 countries in 40 days