What a deal

time to do maths
if one bread in Minecraft costs 3 weat, you gave the villager 20 wheat and he gave you 6 bread
20/3 = 6.67 you were scammed one peice of wheat

joke not intended

Actually I was scammed 2 wheat because it is actually
6x3=18
20-18=2
Or
20/3=18 remainder 2

oh yeah, I did forget that last one