Book Maths Puzzle Percentage

Haveing trouble with this

what is 35% of a number if 12 is 15 % of a number

How do i do this type of equation, and what is it called?

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Book Other Question

A restaurant charges $3.50 for its all you can eat breakfast buffet, and $5.00 for its lunch buffet. At 2 PM
the manager found that 100 people had eaten in the restaurant that day, and the total receipts were $428.How many people had eaten breakfast and how many had eaten lunch? Solve this problem using any method, however to receive full credit, you must specify which method you are using and show supporting work. How many solutions are there normally to a linear equation (x +b = 0)?