I have assumed that the first payment is immediate and the accumulation date is one year after the final smaller payment. You will need to modify my answer if this is not the case.

Accumulated value of regular payments one year after the time of the final payment:

150 * (1.04^(n+1) -1.04)/0.04 * 1.04

Accumulated value of final payment one year after the time of the final payment:

X * 1.04

Simulataneous eqn

150 * (1.04^(n+1) -1.04)/0.04 * (1.04) + X * 1.04 = 2000

X < 150

solving for n

2000/1.04 - 150 * (1.04^(n+1) -1.04)/0.04 < 150

1.51 < 1.04^(n+1)

10.5<n+1

n>9.5

n=10

X = 50.12