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It is simple entertainment problem, designed and intended to be solved MENTALLY, without using equations.
1) compare Monday and Wednesday:
Monday : 2 coffee and 1 doughnut PLUS tip $1.70
Wednesday : 1 coffee and 1 doughnut PLUS tip $1.30
----------------------------------------------------------- The difference is
1 coffee $0.40 ANSWER
2) compare Tuesday and Wednesday:
Tuesday : 1 coffee and 2 doughnut PLUS tip $1.65
Wednesday : 1 coffee and 1 doughnut PLUS tip $1.30
----------------------------------------------------------- The difference is
1 doughnut $0.35 ANSWER
SOLVED.