David and Andy had 81 pencils together. Andy had 3 less pencils than David. Both of them sold some of their pencils. Andy sold 40% more pencils than what David sold. In the end, Andy had half as many pencils left as David. How many pencils had Andy left?
| |
David
(1) |
Andy
(2) |
Make p the same
(2)x2 = (3) |
Comparing David
and Andy at first |
3 more |
|
|
| Before |
42 |
39 |
78 |
| Change |
- 5 u |
- 7 u |
- 14 u |
| After |
2 p |
1 p |
2 p |
Number of pencils that Andy had at first
= (81 - 3) ÷ 2
= 78 ÷ 2
= 39
Number of pencils that David had at first
= 39 + 3
= 42
Number of pencils that Andy sold in percent
= 100% + 40%
= 140%
140% =
140100 =
75 Make p the same.
(1) = (3)
42 - 5 u = 78 - 14 u
14 u - 5 u = 78 - 42
9 u = 36
1 u = 36 ÷ 9 = 4
Number of pencils that Andy had left
= 39 - 7 u
= 39 - 7 x 4
= 39 - 28
= 11
Answer(s): 11
