Answer:
![a.\ 18\ grapefruits\\\\b.\ 6\ grapefruits](https://tex.z-dn.net/?f=a.%5C%2018%5C%20grapefruits%5C%5C%5C%5Cb.%5C%206%5C%20grapefruits)
Step-by-step explanation:
a. Let be "x" the number of grapefruits Carlo buys.
You know that the cost of one grapefruit is $0.80.
Knowing that Carlo buys $14.40 of grapefuit, you can set up the following proportion:
![\frac{1\ grapefruit}{\$0.80}=\frac{x}{\$14.40}](https://tex.z-dn.net/?f=%5Cfrac%7B1%5C%20grapefruit%7D%7B%5C%240.80%7D%3D%5Cfrac%7Bx%7D%7B%5C%2414.40%7D)
Solving for "x", you get:
![\frac{(\$14.40)(1\ grapefruit)}{\$0.80}=x\\\\x=18\ grapefruits](https://tex.z-dn.net/?f=%5Cfrac%7B%28%5C%2414.40%29%281%5C%20grapefruit%29%7D%7B%5C%240.80%7D%3Dx%5C%5C%5C%5Cx%3D18%5C%20grapefruits)
b. Since Kahri spends one-third as much money on grapefruits as Carlo, you need to multiply 18 grapefruits by
in order to calculate the number of grapefruits she buys. Then:
![(18\ grapefruits)(\frac{1}{3})=6\ grapefruits](https://tex.z-dn.net/?f=%2818%5C%20grapefruits%29%28%5Cfrac%7B1%7D%7B3%7D%29%3D6%5C%20grapefruits)