Question: The jumbo bottle of shampoo costs $24.00.  The directions say to use 2 pumps of shampoo and this will give you 120 washings.  If Jackie only uses 1 pump per wash, how much will each pump cost, in cents?
Can you solve this problem?
Answer: 
2 pumps will give you 120 washes so if she only uses 1 pump per wash she doubles the number of washes so she now has 120*2 = 240 washes
The bottle costs $24.00 and she will 240 washes so each wash is 24/240 = $0.10