Sarah had some money in the piggy bank. On Monday, she took 2/5 of the money to buy a present. She had $78 left and she put it back into the piggy bank. On Tuesday, she took 3/8 of the money to buy some story books. She had $28.80 left and she put it back into the piggy bank. After that, she found that there was $246.30 in the piggy bank. How much money did she have in the piggy bank at first?
Answer:
We need to work backwards.
At last she had $246.30. It came from existing saving plus left money $28.80 on Tuesday that she put back to the piggy bank.
246.30 = 5u + 28.80
5u = 217.50, u = 43.50
So, on Tuesday its piggy bank started with 8u:
8u = 8 x 43.50 = 348
The $348 on Tuesday came from existing saving on Monday plus left money $78 on Monday that she put back to the piggy bank.
348 = 3p + 78
3p = 270, p = 90
Hence, on Monday its piggy bank started with 5p:
5p = 5 x 90 = 450.
At first, Sarah had $450 at her piggy bank.
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