**Concept: The question tests on the area of Rates (Time and Work)**

**Solution:**

In 30 hours, the total bolts produced by A and B = 900 + 1500 = 2400

Using the unitary method

2400 bolts -- 30 hours

1600 bolts ---x hours

Therefore 2400/1600 = 30/x

x=30∗1600/2400

=20 hours

**Option E**