4. Four boxes A, B, C and D contained a total of 175 buttons at first. Then the number of buttons in A was increased by 10; the number in B was decreased by 5; the number in C was doubled; and the number in D was halved. As a result, the number of buttons in each box was the same. Find the number of buttons in each box at first.
A [ ] )
B [ ] )
C [ ] ) 175
D [ ] )
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A [ ][ 10 ]
B [ ][5]
C [ ][ ]
D [ ][ ]
< 2 u >
10 u – 1 u – 10 + 5 --> 175
9 u --> 175 + 10 – 5 = 180
1 u --> 180 ÷ 9 = 20
A --> 2 x 20 – 10 = 30 buttons
B --> 2 x 20 + 5 = 45 buttons
C --> 20 buttons
D --> 4 x 20 = 80 buttons
The number of buttons in boxes A, B, C and D were 30, 45, 20 and 80 respectively.
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