Keep doing each step manually, and see where it slips up:
−7.9+8.2−3.4+2.1=−1
−7.9+8.2+(−3.4)+2.1=−1
−7.9+(−3.4)+8.2+2.1=−1
(−7.9+(−3.4))+(8.2+2.1)=−1
−11.3+10.3=−1
Ratios and proportions tie into each other. A ratio in real life would be basically comparing two quantities, as a proportion would be solving for the similar ratio or the equal ratio to another.
The most he can buy for each cupcake is 1.25
