There are 16 red velvet cupcakes.
42 chocolate + 28 red velvet = 70 cupcakes
(42/ 70 *100%)+ (28 / 70 *100%) = (70/70 * 100%)
60 % chocolate + 40% red velvet = 100% cupcake
24 chocolate + X red velvet = Y cupcake
Y cupcake = 24 / 60% = 40 cupcakes
X red velvet = 40 cupcakes * 40% = 16 red velvet.
OR
(24 / 60%) * 40% = 16 red velvet cupcakes