1.5 × 104, 2.6 × 10-2, 3.5 × 102, 6.2 × 10-1 from least to greatest
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Okay first you have to do the math before you can put theme in order:

1.5 × 104 = 156
2.6 × 10 - 2 = 24
3.5 × 102 = 357
6.2 × 10 - 1 = 61.

In order from least to greatest, it'd be
2.6 × 10 -2,
6.2 × 10 - 1,
1.5 × 104,
3.5 × 102.

I hope that helps!