Two bells ring at intervals of 30 minutes and 40 minutes. If they ring together at 7:30 am, when will they ring together again?

Based on the student's work, container A holds 5 litres and container B holds 30 litres.

The total number of litres containers A and B hold is $\boxed{\text{35 litres}}$.

Capacity of container A = 5 litres.
Capacity of container B = 30 litres.

You would need $\boxed{6 \text{ containers}}$ of type A to draw all the water from container B.

The intervals are 30 minutes and 40 minutes.
Prime factorization of 30: $30 = 2 \times 3 \times 5$
Prime factorization of 40: $40 = 2^3 \times 5$
The LCM is found by taking the highest power of each prime factor present:

The bells first rang together at 7:30 am.

The bells will ring together again at $\boxed{\text{9:30 am}}$.

To do this, place the compass point at A, draw an arc that intersects AB. From the intersection point, draw another arc above AB. Repeat from another point on AB to create a second arc that intersects the first, forming a perpendicular line.

Mark the intersection point as D. This ensures AD = 5 cm.

Mark the intersection point as C.

The resulting figure ABCD will be a square with sides of 5 cm.