Darla needs to purchase nine more stamps than envelops. each stamp cost $0.10, and each envelope costs $0.13 . if she pays $2.51, how many stamps and envelops does she buy.

This is a systems of equations.

s = stamp, e = envelope

Substitute e + 9 for s.

.1(e + 9) + .13e = 2.51

Simplify.

.1e + .9 + .13e = 2.51

Simplify.

.14e + .9 = 2.51

Subtract .9 from both sides.

.14e = 1.61

Divide both sides by .14.

e = 11.5

Now, you can input e into the other equation.

11.5 + 9 = s

Simplify.

20.5 = s

s = 20.5, e = 11.5

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step-by-step explanation: