Knowing from general knowledge that carpet in the USA is always priced by the square yard, I immediately noticed the room was very conveniently 3x4 yards = 12 square yards. Which would be 12sy x $10 = $120 less roughly 12 x 50 cents = $6 so ~$114 overall.
If the answers were multiple choice that’d have taken about 7 seconds total.
But I spent the last 35 years doing simple arithmetic like that every few minutes for work.