A student invests 4000, part at 6% and part at 7%. The income from these investments in 1 year is 250. Find the amount invested at 7%.
Solution:
Let I1 be the interest in first case and I2 be interest in second case.
250 = I1 + I2
Let x be the amount invested for 7%.
250 = (4000-x)*6 / 100 + x*7/100
x = 1000.
So the amount invested in 7% is 1000