Answer:
![a.\ 18\ grapefruits\\\\b.\ 6\ grapefruits](https://img.qammunity.org/2020/formulas/mathematics/middle-school/kaj9qf1pjspjm85u5lzaqfhltbywud7kup.png)
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:
![(1\ grapefruit)/(\$0.80)=(x)/(\$14.40)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/dmstvr0sopl1fb60xxwiokbf7yudkv3svx.png)
Solving for "x", you get:
![((\$14.40)(1\ grapefruit))/(\$0.80)=x\\\\x=18\ grapefruits](https://img.qammunity.org/2020/formulas/mathematics/middle-school/u4ntk9qvftv60i2mboj6xv195sxohcoy3y.png)
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)((1)/(3))=6\ grapefruits](https://img.qammunity.org/2020/formulas/mathematics/middle-school/24s6ipatp9vx0iyrl1rlw44huq9akxxlkm.png)