Write a general formula to describe the variation:
s varies jointly with the inverse of the square p and the inverse of the cube l;
s=4 when p=2 and l=5

Step 1:Write the correct equation. Combined variation problems are solved using a combination of direct variation (y = kx), inverse variation , and joint variation (y = kxz) equations. When dealing with word problems, you should consider using variables other than x, y, and z, you should use variables that are relevant to the problem being solved. Also read the problem carefully to determine if there are any other changes in the combined variation equation, such as squares, cubes, or square roots.Step 2:Use the information given in the problem to find the value of k, called the constant of variation or the constant of proportionality.Step 3:Rewrite the equation from step 1 substituting in the value of k found in step 2.Step 4:Use the equation found in step 3 and the remaining information given in the problem to answer the question asked. When solving word problems, remember to include units in your final answer.