Model an inheritance using principal and simple interest

[Banking] Stephen inherited $50,000 and put part in an investment-savings account, part in bonds and the rest in an investment fund. After one year he received a total of $3,240 in simple interest. The account paid 6% annually, bonds 7% and the fund 8%. There was $12,000 more invested in bonds than in the fund. Find the amount invested in each category.