National Council of Teachers of Mathematics
Nctm: Illuminations: Volt Meter
As you put the different type of batteries on the volt meter pay attention to the voltage equations in the table. See how positive and negative numbers interact.
Varsity Tutors
Varsity Tutors: Hotmath: Algebra Review: Add and Subtract With Negatives
Find a quick, concise explanation of how to add and subtract with negatives. Step by step examples are included to help you understand how to find the sum or difference when negative numbers are involved.