Page 112 - ICSE Math 7

Example 12: A line segment of length 2.8 m is measured 2.9 m by mistake. Find the error percent.
Solution: Error = 2.9 m – 2.8 m = 0.1 m
Error 0.1 4
Error per cent = × 100% = × 100% = 3 %
Actual length 2.8 7

EXERCISE 8.2

1. If a number increases from 650 to 715, then find the increase in percentage.
2. If 400 decreases to 380, then find the decrease in percentage.
3. Jayant reduces his weight from 84 kg to 77 kg. Find the percentage of reduction.
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4. A sum of ` 1,200 increases by 7 %. The resulting amount then decreases by 25%. Find the final
amount. 3
5. A number after increasing by 20% is 1,500. Find the original number.
6. Shruti wants to increase her weight from 40 kg to 45 kg. Find the increase in percentage.
7. Seema’s monthly salary increases from ` 50,700 to ` 53,911. Find the increase in percentage.
8. Sumesh’s height is 147 cm after an increase of 5%. Find his earlier height.
9. Anu’s actual weight is 50 kg, but she is wrongly weighed as 52 kg. Find the error percentage.
10. A’s income is 30% more than B’s. How much per cent is B’s income less than A’s?

AT A GLANCE

¾ Per cent means ‘out of hundred’ or ‘for every hundred’. It is denoted by the symbol ‘%’.
¾ To convert a fraction or a decimal into percentage, multiply it by 100 and put the symbol of
percentage.
¾ To convert percentage into a fraction or a decimal, remove the symbol of percentage and divide
it by 100.
¾ To express one quantity as a percentage of the other, divide the first quantity by the second and
then multiply by 100%.
¾ To find the percentage of a given quantity, express the given per cent as a fraction of 100 and
multiply by the given number.
¾ To find the number from a given percentage of the number, write the given percentage as a
fraction of 100 and then divide the given number by this fraction.
Increase in value
¾ Increase in percentage = × 100%
Original value
Decrease in value
¾ Decrease in percentage = × 100%
Original value
¾ Increased new value = Original value (1 + percentage as a fraction)
¾ Decreased new value = Original value (1 – percentage as a fraction)

MENTAL MATHS

1. Write True or False.

(a) 34.5 out of 100 is 3.45% (b) 40% of 100 = 20% of 200
(c) 5 cm = 5% of 1 m (d) 27 g = 27% of 1 kg
(e) 10 mL = 1% of L (f) 17 p = 17% of a rupee

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