Hobby electronics
The eight sums a breadboard keeps asking for, each ending at a value you can actually buy.
Ohm's law
Voltage, current, resistance and power from any two of them.
OpenLED series resistor
Series resistor, nearest stocked value and the power it dissipates.
OpenVoltage divider
Divider output, current, output impedance and the resistor for a target.
OpenRC filter cutoff
Cutoff, time constant, rise time and the part for a target frequency.
Open555 timer
Astable frequency and duty cycle, or monostable pulse width.
OpenPCB trace width
Trace width to IPC-2221, with resistance, drop and power lost.
OpenBattery life
Runtime from cell capacity, duty cycle and sleep current.
OpenResistor colour code
Resistor colour bands in both directions, with tolerance.
OpenThese are planning figures, not a substitute for a standard, a specification or a professional. Check anything that matters against the source your trade actually works to.