Substitution questions
A table says, for example: + means ÷, − means ×, × means +, ÷ means −. Then
18+6−3×4÷2
is read by replacing each symbol first:
18÷6×3+4−2=3×3+4−2=11
Interchange questions
Two forms appear:
- "If + and × are interchanged, which equation is correct?" — swap those two symbols wherever they appear in each option and test it.
- "Interchange which two signs makes the equation correct?" — try each pair in turn; there are only six pairs among four operators, and usually the first or second works.
A third variant swaps two numbers rather than two signs. Test the options rather than solving in general: it is faster.
Balancing an equation
The question gives an equation with blanks and asks which set of signs fills them, such as 12 ? 4 ? 2=10. Work backwards from the answer: 12÷4=3 gives no 10; 12−4+2=10 ✓. Test option by option, and remember BODMAS applies to your filled-in version too.
Defined operations
A rule defines a new operator: a∗b=a2−b, or aΔb=2a+b. Substitute the numbers into the definition exactly as written, taking care with the order — most defined operations are not symmetric, so a∗b=b∗a.
For nested expressions, evaluate the innermost bracket first: (3∗2)∗4 means find 3∗2, then apply ∗ with 4.
Inserting symbols
"Insert arithmetic signs to make 6 _ 3 _ 2=4 true." Try division first (it constrains most), then multiplication: 6÷3+2=4 ✓.
Worked examples
✎Worked ExampleExample 1 — plain substitution
If + means ×, − means +, × means ÷ and ÷ means −, find 16+4÷2−8×4.
Solution. Substituting: 16×4−2+8÷4=64−2+2= 64.
✎Worked ExampleExample 2 — BODMAS after substitution
If × means + and + means ÷, find 12×6+3.
Solution. 12+6÷3=12+2= 14 — division is done before addition even though it was written second.
✎Worked ExampleExample 3 — which equation is correct
If + and ÷ are interchanged, which is correct?
(a) 16÷4+2=10 (b) 18+3÷6=12
Solution. Swapping the two symbols in (a): 16+4÷2=18=10. In (b): 18÷3+6=12 ✓ → (b).
✎Worked ExampleExample 4 — which signs to interchange
Which two signs should be interchanged to make 9+3−4×2=19 correct?
Solution. As printed the left side is 9+3−8=4. Test the pairs:
- + and ×: 9×3−4+2=25 ✗
- + and −: 9−3+4×2=14 ✗
- − and ×: 9+3×4−2=19 ✓
So − and × are interchanged. With only three pairs to try among three signs, testing is quicker than reasoning.
✎Worked ExampleExample 5 — a defined operation
If a∗b=a2−b, find 5∗3 and 3∗5.
Solution. 5∗3=25−3= 22; 3∗5=9−5= 4. The operation is not symmetric.
✎Worked ExampleExample 6 — a nested defined operation
With aΔb=2a+b, find (8Δ4)Δ6.
Solution. 8Δ4=6; then 6Δ6= 6.
✎Worked ExampleExample 7 — inserting signs
Insert signs to make 9 _ 3 _ 2=5 true.
Solution. 9÷3+2= 5 ✓.
✎Worked ExampleExample 8 — interchanging numbers
Which two numbers should be interchanged to make 8+6×4−2=30 correct?
Solution. As printed: 8+24−2=30 ✓ — the equation is already correct, so no interchange is needed. Check the given equation before hunting for a swap.
✎Worked ExampleExample 9 — mixed symbols
If A means +, B means −, C means × and D means ÷, find 15 A 12 D 4 C 3 B 6.
Solution. 15+12÷4×3−6=15+9−6= 18.