Use subtraction to find unknown angle measures in problems involving adjacent angles and known whole angles.
Find the Missing Angle — Subtraction Practice
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The printable gives you a whole angle and one of its parts, twelve times over. The arithmetic is the easy half. The half worth learning is the check, because it lets you catch your own mistake without an answer key.
The rule, and the check
whole − known part = missing part. Two adjacent angles share a vertex and a side, and together they make the whole angle.
Now check it. Add your answer back to the part you were given. You must land exactly on the whole angle. If you do not, the subtraction slipped — and you have found that out by yourself.
One more check you can do by eye: the missing part is always smaller than the whole angle. An answer bigger than the whole is wrong before you calculate anything.
Question 1
A whole angle of 90° is split into two adjacent angles. One part is 35°. How big is the other?
55°. 90° − 35° = 55°. Check it: 35° + 55° = 90°, which is the whole angle — so it is right.
Question 2
A whole angle of 120° is split into two adjacent angles. One part is 48°. How big is the other?
72°. 120° − 48° = 72°. Check it: 48° + 72° = 120°, which is the whole angle — so it is right.
Question 3
A whole angle of 160° is split into two adjacent angles. One part is 95°. How big is the other?
65°. 160° − 95° = 65°. Check it: 95° + 65° = 160°, which is the whole angle — so it is right.
Question 4
A whole angle of 75° is split into two adjacent angles. One part is 18°. How big is the other?
57°. 75° − 18° = 57°. Check it: 18° + 57° = 75°, which is the whole angle — so it is right.
Question 5 — select all that apply
Use the check. Select every line below that is correct, then press Check.
The three correct ones are 90° − 35° = 55°, 100° − 27° = 73°, 75° − 18° = 57°. The others each fail the check: 48° + 82° = 130°, not 120°; 65° + 95° = 160°, not 150°; 39° + 69° = 108°, not 98°. Notice that all three mistakes are off by exactly ten — a slip in the tens column is the commonest way to get one of these wrong.
Question 6 — put them in order
Click these in order, from the smallest missing angle to the largest.
80° − 26° = 54° < 85° − 29° = 56° < 100° − 27° = 73° < 145° − 57° = 88°. The biggest whole angle does not give the biggest missing part — 100° − 27° starts from 100° and gives only 73°, because the part already taken is large.
Think about it
Why does adding your answer back to the known part prove you were right?
A whole angle is 120° and one part is 48°. Could the missing part be 130°? How do you know without subtracting?
If the two parts are equal and the whole is 140°, what is each part?
Make up a question of your own like these, and write the check underneath it.