Answer:
20: But here is how.
Logically. <Which as you practice means you won’t have to write anything for this type of problem =) >
“Every shirt could come with any one of the pants.” (notation: sn is shirt number n)
s1 with 4 pants, so 4 outfits
s2 with 4 pants, so 4 outfits
s3 with 4 pants, so 4 outfits
s4 with 4 pants, so 4 outfits
s5 with 4 pants, so 4 outfits
Total of 5×4