Let e = the cost of one egg and s = the cost of one sandwich.

Since the cost of 12 eggs is between 0.9 and 1.20,
0.9 < 12e < 1.20

Divide by 3 to find where the cost of 4 eggs lies:
0.3 < 4e < 0.4

Since the cost of 5 sandwiches is between 10 and 15,
10 < 5s < 15

Divide by 5 and multiply by 3 to find where the cost of 3 sandwiches lies:
6 < 3s < 9

Add the costs of 4 eggs and 3 sandwiches:
6+0.3 < 4e + 3s < 9+0.4
6.3 < 4e + 3s < 9.4