In math
"of" means times (x)
"is" means equals (=)
"a number" is your unknown which you can call anything but we'll call x
2/5 of a number is 14
Translation...
2/5 * x = 14
Divide each side by 2/5 to isolate x. To divide a fraction, multiply the reciprocal of the divisor (flip the numerator and denominator).
x = 14 * 5/2
x = 70/2
x = 35
2/5 of 35 is 14.