The exponent high school Drama club plan to sell memo pads and ball pen at their presentation to raise money for their outreach programs. They plan to sell a total of 120 items, and they need to make 8400 to have sufficient money for the program. How many memo pads and ball pens do they need to sell if the cost of memo pads is P8.00 each and the cost of ball pen is P5.00 each

This is a system of two equations with two unknowns.

Let m=number of memo pads and let p=number of pens.

They must make $8400:

8m+5p=8400

They want to sell 120 items:

m+p=120

There are two equations with two unknowns.

Solve for m and p.

It takes 6 hours for pump A, used alone, to fill a tank of water. Pump B used alone takes 8 hours to fill the same tank. We want to use three pumps: A, B and another pump C to fill the tank in 2 hours. What should be the rate of pump C? How long would it take pump C, used alone, to fill the tank?

Use this:

\frac{1}{A} + \frac{1}{B} + \frac{1}{C} = \frac{1}{A, B, C}

Where A means the time for A alone,
B means the time for B alone,
C means the time for C alone,
A, B, C means the time when A, B and C together,

take to fill the tank.