Your cast is appled to tew filter (where condition) not to the output (select clause)
so try
SELECT CAST(WIDTH AS DECIMAL) W, CAST(BREADTH AS DECIMAL) B
FROM orders
WHERE ODR_ID ='3573'
AND CAST(WIDTH AS DECIMAL) = CAST('1.500' AS DECIMAL)
AND CAST(BREADTH AS DECIMAL) = CAST('0.500' AS DECIMAL)
ORDER
BY ID DESC
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