Start by solving for S:

Subtracting 100 from both sides:

Multiplying through by :

Multiplying through by the denominator:

Dividing both sides by :

Subtracting from both sides:

Writing everything over a common denominator:

Simplifying:

, so:

Which means that:

, so:

See if you can take it from here, using a similar approach. Don't substitute for L until you've solved for