The simple interest I (in dollars) for a savings account is jointly proportional to the product of time (in years) and the principal P (in dollars). After 11 months the interest on a principal of $3970 is $86. What is the constant of the variation

Simple interest I is jointly proportional to the product of time in years and the principal, therefore; mathematically;

I α Pt

introducing a constant;

I = kPt; but I = $ 86, P = $ 3970, while t = 11/12 years

Thus; k = I/Pt

= 86 / 3970 (11/12)

= 0.0236

Therefore, the constant of the variation is ≈ 0.024