What is the size of the basis for cubic B - splines

Question

I have an exam questions which says we have mortality data observed between ages 20 and 60 with knots at 20, 30, 40, 50 and 60. Then the questions say we will use a set of 7 basis splines to fit a cubic spline to this data. I'm quite confused about this as I thought the number of basis splines needed was the number of knots you have, k = 5 here, plus the degree of the polynomial so 3 here, so I thought it would be 8 basis splines not 7 needed?

Picture of question below

Model 4 (D_x is amount of deaths at age X, E^c_x is exposure to risk at age X, B_j(X) is a set of basis spline functions, theta_j are coefficients)

Answer 1

Number of knots = number of basis functions + order of spline.

The order of the spline is its degree plus 1.

You are told that the knots are 20, 30, 40, 50, 60, but you’re not told anything about their multiplicities.

But, TBH, the question doesn’t make sense to me, either.