Joan weighs 10 pounds less than her older sister. The average of the two sisters’ weights is 85 pounds. How much does Joan’s older sister weigh?

Joan's older sister weighs 90 pounds

Step-by-step explanation:

x = older sister

x - 10 = Joan

(x + (x-10))/2 = 85

2x - 10 = 170

2x = 180

x = 90

90 pounds

Step-by-step explanation:

We can set up an equation to find their weights.

Let's start by naming Joan's weight x.

Her sister's weight would then be x+10, since Joan weighs 10 pounds less.

To find the average between 2 numbers, you need to add them together, then divide by 2.

So we can set up the following equation:

(x+x+10)/2=85

Now let's isolate x.

We can first multiply both sides by 2.

x+x+10=170

Combine like terms.

2x+10=170

Subtract 10 from both sides.

2x=160

Subtract both sides by 2.

x=80

Joan weighs 80 pounds.

x+10 is her sister's weight.

80+10=90

Joan's older sister weighs 90 pounds.