How many cars have a spare tire?

Question

In a parking lot, some cars have 1 spare tire and others have no spare tires. All together, there are 43 tires and 9 cars. How many cars have a spare tire?

Answer 1

Hi Debbie;

Each car has four tires which are NOT spares.

(43 tires) - {(9 cars)[(4 tires)/(1 car)]}

In the right bracketed equation, the unit of cars is in the numerator and denominator.  It cancels...

(43 tires) - {(9 cars)[(4 tires)/(1 car)]}

(43 tires) - [(9)(4 tires)]

The only unit remaining is tires.  This is what we want...

(43 tires) - (36 tires)

7 tires

7 tires are not mounted, but are spares.

7 cars have one spare.