Increase & Decrease Practice — 21 Speed Drills

This lesson is pure practice. For each number, calculate both the increased and decreased values. Use the ratio/fraction method — no calculators!

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How to Solve Each Row

For each row you get a number, an increase %, and a decrease %. You need to find:

• (a) Number after increase
• (b) Number after decrease

Method: Convert % to fraction → find multiplier → multiply.

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The 21 Drills

#	Number	Increase by	(a) After Increase	Decrease by	(b) After Decrease
1	60	10% (1/10)	66	20% (1/5)	48
2	80	25% (1/4)	100	75% (3/4)	20
3	400	38% (19/50)	552	14% (7/50)	344
4	48	33.33% (1/3)	64	62.50% (5/8)	18
5	84	14.28% (1/7)	96	16.67% (1/6)	70
6	52	7.69% (1/13)	56	25% (1/4)	39
7	240	125% (5/4)	540	83.33% (5/6)	40
8	90	16.67% (1/6)	105	91.67% (11/12)	7.5
9	88	36.36% (4/11)	120	75% (3/4)	22
10	108	41.67% (5/12)	153	22.22% (2/9)	84
11	120	62.5% (5/8)	195	91.67% (11/12)	10
12	160	162.5% (13/8)	420	12.5% (1/8)	140
13	75	12% (3/25)	84	26.67% (4/15)	55
14	1280	75% (3/4)	2240	90% (9/10)	128
15	630	77.77% (7/9)	1120	42.86% (3/7)	360
16	960	12.5% (1/8)	1080	8.33% (1/12)	880
17	136	5.88% (1/17)	144	37.5% (3/8)	85
18	90	6.66% (1/15)	96	5.56% (1/18)	85
19	225	26.67% (4/15)	285	55.55% (5/9)	100
20	176	37.50% (3/8)*	110†	90.91% (10/11)	16
21	112	28.57% (2/7)	144	92.86% (13/14)	8

†Row 20 (a): 176 × (−37.5%) = 176 × 5/8 = 110. This is actually a decrease of 37.5%, yielding 110.

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Detailed Solutions for Select Rows

Row 1: Number = 60

(a) +10%: 10% = 1/10. Multiplier = 11/10. → 60 × 11/10 = 66

(b) −20%: 20% = 1/5. Multiplier = 4/5. → 60 × 4/5 = 48

Row 5: Number = 84

(a) +14.28%: 14.28% = 1/7. Multiplier = 8/7. → 84 × 8/7 = 96

(b) −16.67%: 16.67% = 1/6. Multiplier = 5/6. → 84 × 5/6 = 70

Row 7: Number = 240

(a) +125%: 125% = 5/4. Multiplier = 1 + 5/4 = 9/4. → 240 × 9/4 = 540

(b) −83.33%: 83.33% = 5/6. Multiplier = 1/6. → 240 × 1/6 = 40

Row 9: Number = 88

(a) +36.36%: Need increase to get 120.

• 88 → 120: change = +32. 32/88 × 100 = 4/11 × 100 = 36.36% ✓
• 36.36% = 4/11. Multiplier = 15/11. → 88 × 15/11 = 120

(b) −75%: 75% = 3/4. Multiplier = 1/4. → 88 × 1/4 = 22

Row 15: Number = 630

(a) +77.77%: 77.77% = 7/9. Multiplier = 16/9. → 630 × 16/9 = 1120

(b) −42.86%: 42.86% = 3/7. Multiplier = 4/7. → 630 × 4/7 = 360

Row 21: Number = 112

(a) +28.57%: 28.57% = 2/7. Multiplier = 9/7. → 112 × 9/7 = 144

(b) −92.86%: 92.86% = 13/14. Remaining = 1/14. → 112 × 1/14 = 8

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Speed Challenge

Try to solve each row in under 10 seconds. The key skills:

1. Instant fraction recognition — see 16.67%, think 1/6
2. Quick multiplier formation — +1/6 means × 7/6, −1/6 means × 5/6
3. Clean mental multiplication — look for numbers divisible by the denominator

Target: Complete all 21 rows in under 4 minutes. If you can do this, percentage calculations will never slow you down in exams.