Answer:
4.5 birds, 7.5 mice
Explanation:
We know that birds have 2 legs and mice have 4. Furthermore, birds have 1 head as well as mice.
If b represents the amount of birds and m represents the amount of mice, because there are only birds and mice here, the total amount of birds and mice (b + m) must equal 12.
b + m = 12
Next, to find the total amount of bird legs, we must add 2 legs for each bird. Another way of putting this is to multiply 2 by the amount of birds, or b * 2. Similarly, the amount of mice legs is equal to m * 4. The total amount of legs is equal to 39, so
2b + 4m = 39
Our two equations are thus
b + m = 12
2b + 4m = 39
One way to solve this is to solve for m in the first equation and plug that into the second. We have
b + m = 12
subtract b from both sides
m = 12-b
substitute that into the second equation
2b + 4(12-b) = 39
2b + 48 - 4b = 39
-2b + 48 = 39
subtract 48 from both sides to isolate the b and its coefficient
-2b = -9
divide both sides by -2 to isolate b
b = 4.5
Therefore, there are 4.5 birds on the ground (there is somehow a half of a bird!)
Plugging that into m = 12-b, we have m = 12-4.5 = 7.5