Answer:
3
Step-by-step explanation:
Given that installing 6 apps increases the Eric : Ian app ratio from 3:4 to 5:4, you want to know how many more apps Ian needs to install to have the same number as Eric.
We note that the number of apps representing Ian's apps remains at 4 in both ratios, but increasing Eric's apps by 6 changes his number of ratio units from 3 to 5, a change of 2 ratio units. That is, each ratio unit represents 3 apps.
Eric now has 1 more ratio unit (5) than Ian (4), so Ian needs to increase his number by 1 ratio unit, or 3 apps.
Ian must install 3 apps to have the same number as Eric.
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Additional comment
To see how many actual apps we're talking about, multiply the ratios by 3. The initial Eric : Ian app ratio was 9 : 12. After Eric installed 6, it became (9+6) : 12 = 15 : 12. So, Ian needs to install 15-12 = 3 apps to have as many as Eric.
