The problem statement is not clear regarding which money is invested. We will assume it is the 80% of $8750 that is set aside for future education. That amount is
... 0.80 × $8750 = $7000
If x represents the amount invested at 7%, then the total interest earned is
... (7000 -x)×4% + (x)×7% = 420
... 280 +0.03x = 420
... x = 140/0.03 ≈ 4666.67
$4666.67 was invested at 7%.
$2333.33 was invested at 4%.