Work this out by first finding how many milligrams sit in each litre, then converting that mass into milliequivalents using the definition of an equivalent.
An equivalent weight is the molecular weight divided by the valence involved in the reaction. For NaCl, the molecular weight is 23 (Na) + 35.5 (Cl) = 58.5, and the ion exchange valence is 1, so one equivalent of NaCl weighs 58.5 g, and one milliequivalent weighs 58.5 mg.
Since the concentration is 1300 ppm, that is the same as 1300 mg of NaCl in every litre of solution. Convert that mass into milliequivalents by dividing by the weight of one milliequivalent:
\[\frac{1300\ \text{mg/l}}{58.5\ \text{mg/meq}} = 22.22\ \text{meq/l}\]
This confirms the same figure as the direct ppm-over-equivalent-weight method, 22.22 meq/l.
\[\boxed{22.22\ \text{meq/l}}\]