Vaidev started saving part of his pocket money by putting 2 coins in a money box every day. Each coin was either a 10¢ or 20¢ coin. His grandmother also put on a $1 coin in the box every 7 days. The total value of the coins after 70 days was $31.30.
- How many coins were there altogether?
- How many of the coins were 20¢ coins?
Total nuber of coins that his grandmother put in
= 70 ÷ 7
= 10
Total number of coins that Vaidev saved
= 2 x 70 = 140 Total number of coins
= 10 + 140
= 150
(b)
Amount contributed by his grandmother
= 10 x 1
= $10
Amount that Vaidev saved
= 31.30 - 10
= $21.30
$1 = 100¢
$21.30 = 2130¢
Number of
20¢ coins |
Total value
of 20¢ coins |
Number of
10¢ coins |
Total value
of 10¢ coins |
Total value |
140
|
140 x 20
= 2800¢ |
0
|
0
|
2800¢
|
139
|
139 x 20
= 2780¢ |
1
|
1 x 10 =
10¢ |
2790¢
|
| 73 |
73 x 20
= 1460¢ |
67 |
67 x 10 =
670¢ |
2130¢
|
Amount that Vaidev saved if all he saved were 20¢ coins
= 140 x 20
= 2800
Big difference between the total values of 20¢ coins and 10¢ coins
= 2800 - 2130
= 670
Small difference between the value of 20¢ coins and 10¢ coins
= 20 - 10
= 10
Number of 10¢ coins
= 670 ÷ 10
= 67
Number of 20¢ coins
= 140 - 67
= 73
Answer(s): (a) 150; (b) 73
