Question

A contractor mixes concrete from bags of premix for small jobs. how many bags with 5% cement should he mix with 4 bags of 13.5% cement to produce a mix containing 7% cement

Answer 1

Imagine all bags weigh 100 lb.
That means a 13.5% bag has 13.5 lb of cement.
Then similarly a 5% bag has 5 lb of cement,
and a 7% bag has 7 lb of cement.

You are using 4 bags of 13.5%. That means 4 * 13.5 lb of cement.
Now you need to add x number of 5% bags.
x bags of 5% cement will have 5x lb of cement.

Ok so far?