3.
3 red birds for 4 days eat 36 grams of seed, 5 blue birds for 3 days eat 60 gram of seed.
For how many days could be feed 2 red birds and 4 blue birds with 88 gr seed?
1. **Calculate the daily seed consumption for one red bird:**
   - Given: 3 red birds eat 36 grams of seed in 4 days.
   - Total seed consumption for 3 red birds in 4 days: 36 grams.
   - Seed consumption for 1 red bird in 4 days: 
     \[
     \frac{36 \text{ grams}}{3} = 12 \text{ grams}
     \]
   - Seed consumption for 1 red bird in 1 day:
     \[
     \frac{12 \text{ grams}}{4} = 3 \text{ grams}
     \]

2. **Calculate the daily seed consumption for one blue bird:**
   - Given: 5 blue birds eat 60 grams of seed in 3 days.
   - Total seed consumption for 5 blue birds in 3 days: 60 grams.
   - Seed consumption for 1 blue bird in 3 days:
     \[
     \frac{60 \text{ grams}}{5} = 12 \text{ grams}
     \]
   - Seed consumption for 1 blue bird in 1 day:
     \[
     \frac{12 \text{ grams}}{3} = 4 \text{ grams}
     \]

3. **Calculate the combined daily seed consumption for 2 red birds and 4 blue birds:**
   - Daily seed consumption for 1 red bird: 3 grams.
   - Daily seed consumption for 2 red birds:
     \[
     2 \times 3 \text{ grams} = 6 \text{ grams}
     \]
   - Daily seed consumption for 1 blue bird: 4 grams.
   - Daily seed consumption for 4 blue birds:
     \[
     4 \times 4 \text{ grams} = 16 \text{ grams}
     \]
   - Total daily seed consumption for 2 red birds and 4 blue birds:
     \[
     6 \text{ grams} + 16 \text{ grams} = 22 \text{ grams}
     \]

4. **Determine the number of days 88 grams of seed will last:**
   - Total seed available: 88 grams.
   - Daily seed consumption for 2 red birds and 4 blue birds: 22 grams.
   - Number of days the seed will last:
     \[
     \frac{88 \text{ grams}}{22 \text{ grams/day}} = 4 \text{ days}
     \]

The final answer is $\boxed{4 \text{ days}}$