A part-whole relationship is the cue when a total is split into two nonoverlapping groups. Write the known part plus the unknown part equals the total, then subtract the known part. The total includes both groups; it is not a third group to add. On a timed test, one subtraction is quicker than opening Desmos.
Hints
- Hint 1
In a part-whole relationship, the counts of the two groups add to the total. Let count the mammals. How would you use the bird count and to write that total?
- Hint 2
To find a missing part, subtract the known part from the whole. Adding the known part to the total would give you more than the whole group.
Step-by-step
Subtract the known group
Step 1Write the two groups as one total
No Desmos needed. The missing group takes one subtraction. Let be the number of mammals. Every animal is in one of the two groups, so the birds and mammals add to the animals:
- Step 2
Remove the bird count
Subtract from both sides to find the missing group:
- Step 3
Count the mammals
Compute the difference:
Since counts mammals, the rescue cared for 13 mammals. Choice D.