Answer
$2^{0},2^{1},2^{2},2^{3},2^{4},2^{5},2^{6},2^{7},2^{8},2^{9},2^{10},2^{11},2^{12},2^{13},2^{14},2^{15},2^{16}$
Work Step by Step
If a cell divides in two every 15 minutes, then in 4 hours there will be $\dfrac{4\cdot60}{15}=16$ divisions.
Then the number of cells every 15 minutes will be the sequence:
$$1,2,4,8,16,32,64,128,256,512,1024, .. .$$
It's easier to represent these numbers as powers of two.
$2^{0},2^{1},2^{2},2^{3},2^{4},2^{5},2^{6},2^{7},2^{8},2^{9},2^{10},2^{11},2^{12},2^{13},2^{14},2^{15},2^{16}$
