(a) 12/8 = m/2, (b) 140/36 = q/9, (c) 21/p = 7/3, (d) 6/12 = 2/n, (e) 26/18 = 13/k, (f) 52/c = 39/21, (g) 52/c = 39/21, (h) b/24 = 4/32, (I) 18/d = 21/140, (j) 616/y = 504/36, (k) 781/55 = 639/h

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What you have to do basically is to multiply/divide both sides to get the variable alone on one side.

Take (k) for example:

\frac{781}{55} = \frac{639}{h}

Multiply both sides by h, what do you get?

\frac{781}{55} \times h= \frac{639}{h} \times h

\frac{781h}{55}= \frac{639}{\cancel{h}} \times \cancel{h}

\frac{781h}{55}= 639

Now, multiply by 55, then divide by 781;

\frac{781h}{55} \times \frac{55}{781}= 639\times \frac{55}{781}

\frac{\cancel{781}h}{\cancel{55}} \times \frac{\cancel{55}}{\cancel{781}}= 639\times \frac{55}{781}

h = 639\times \frac{55}{781}

Now you can simplify that.

Can you try to solve the others and post what you get for them? :)