A subscriber of http://www.ilearnbywatching.com asked:

Hi! My daughter has been watching some of the FRP videos and the most recent one she is looking at is the P6 Set B Rev Up. She realised that "cross multiplication" occurred many times. She does not understand why she must cross multiply. Is that a special method? How does it work? I would be grateful if you could help us. Thank you very much! Warm regards Kelly :)

Our reply:

–This part of the working for our students will be the mechanical part of the working after the set up. A quick way to understand is: 1u + 10 / 5u – 30 = 1/3

So essentially you are cross multiplying the above.

–You could understand it through equality …meaning: 2/3 = 4/6, cross multiply you get 6X2 = 3X4 . So cross multiplication holds true.

If you are drawing bar models, you could explain it with parts and units. So 3 of (1u + 10) equals 1 of (5u – 30). You can draw the 1u as a rectangular block.

Thank you so much!! My daughter understands the bar model explanation. She says she now knows how to use the cross multiplication method. Thank you and the teacher very much for helping her. Warm regards & with gratitude Kelly :) We are happy to reply to our subscribers.